Biology 2 AP  Chapter 29 Notes

 

Invertebrates and the Origin of Animal Diversity

 

Over one million species of animals are living today; 95% of these are invertebrates.

·        Grouped into about 35 phyla depending on the taxonomic view followed.

·        Most are aquatic

·        The most familiar belong to the subphylum Vertebrata of the phylum Chordata. This is only about 5% of the total.

 

I.          What is an animal?

 

            Although there is a great animal diversity, most animals share the following         features:

·        Multicellular, eukaryotic organisms.

            à Heterotrophy is by ingestion.

            Ingestion: Eating other organisms or decomposing organic matter           (detritus). This mode of nutrition distinguishes animals from the plants      and fungi.

·        Carobhydrate reserves are stored as glycogen.

·        No cell walls are present; animals do have intercellular junctions presents: desmosomes, gap junctions, and tight junctions.

·        Highly differentiated body cells which are organized into tissues, organs and organ systems for such specialized functions as digestion, internal transport, gas exchange, movement, coordination, excretion, and reproduction.

·        Nervous tissue (impulse conduction) and muscle tissue (movement) are unique to animals.

·        Reproduction is typically sexual with flagellated sperm fertilizing nonmotile eggs to form diploid zygotes. A diploid stage dominates the life cycle.

            à The zygote undergoes a series of mitotic divisions known as cleavage            which produces a blastula in most animals.

            à Gastrulation occurs after the blastula has formed; during this process,            the embryonic forms of adult body tissues are produced.

                        à Development in some animals is direct to maturation while the life                              cycle of others includes larvae which undergo metamorphosis into                                             sexually mature adults.

 

            The seas contain the greatest diversity of animal phyla, although many groups live           in fresh water and terrestrial habitats.

 

II.        Comparative anatomy and embryology provide clues to animal phylogeny: an    overview of animal diversity.

 

            Animals diversified so rapidly during the late Precambrian and early Cambrian    periods that it is difficult to derermine the exact sequence of branching from the       fossil record.

·        To reconstruct the evolutionary history of the animal phyla, zoologists use information from comparative anatomy, embryology of living animals, and molecular systematic.

·        Most zoologists agree that the animal kingdom is monophyletic and the ancestral organism was probably a choanoflagellate.

 

            A.        The Parazoa-Eumetazoa Split

 

                        The Parazoa contains onlt the Porifera, which:

·              Have unique development and simple anatomy that separates them from            the other animals.

·              Lack true tissue.

 

                        The Eumetazoa include animals with true tissues, the presence of which              has permitted the evolution of a more complex anatomy.

           

 

            B.        The Radiata-Bilateria Split

                       

                        The division of eumetazoams into two branches is based partly on body                                     symmetry.

·        The Radiata exhibit radial symmetry.

            à These animals have an oral (top) and aboral (bottom) side, but no front,        back, left, or right side.

·        The Bilateria exhibit bilateral symmetry.

            à Bilaterally symmetrical animals have dorsal, ventral, anterior,             posterior, left and right body surfaces.

            à These animals exhibit cephalization (an evolutionary trend toward      concentration of sensory structures at the anterior end.)

           

            Care must be taken when assigning an animal to an evolutionary line as                          symmetry may change between the larval and adult forms. The phylum             Echinodermata shows a secondary radial symmetry in adults, which evolved as an    adaptation to their sedentary lifestyles. They are actually in the bilateria.

 

            Examination of development and body plan can define the radiate-bilateria split better than symmetry.

 

·        The early embryo of all eumetazoans undergoes gastrulation. Concentric germ layer develops which form the various tissues and organs as development continues.

            à The Radiata develop only two germ layers (ectoderm and endoderm)           and are termed deploblastic

            à The Bilateria develop three germ layers (ectoderm, endoderm, and   mesoderm) are termed triploblastic.

 

The Germ Layers of an Early Embryo:

 

Ectoderm

·        Covers the surface of the embryo.

·        Forms the animal’s outer covering and the central nerous sustem in some phyla.

 

            Endoderm

·        Innermost germ layer which lines the archenterons (primitive gut).

·        Forms the lining of the digestive tract, and outpocketings give rise to the liver and lings of vertebrates.

 

            Mesoderm

·        Located between the ectoderm and endoderm in triploblastic animals.

·        Forms the muscles and most organs located between the digestive tract and outer covering of the animal.

 

 

C.                 The Acoelomate-Coelomate Split

 

            Triploblastic animals can also be grouped on the basis of whether a body           cavity develops or not and, if so, how the cavity develops.

           

            Animals in which no body cavity develops are termed acoelomate.

 

            Acoelomate: An animal body plan characherized by no body cavity       present between the digestive tract and the outer body wall.

·        The area between the digestive tract and outer wall is filled with cells producing a solid body.

 

            Animals in which a body cavity develops may be termed pseudocoelomate        or coelomate, depending on how the cavity develops.

 

            Pseudocoelomate : Animal body plan characterized by a fluid-filled body           cavity that separates the digestive tract and the outer body wall.

·        This cavity is not completely lined with tissue dervised from mesoderm.

 

            Coelomate: Animal body plan characterized bu a fluid-filled body cavity             completely lined with tissue derived from mesoderm that separates the         digestive tract from the outer body wall.

 

            The functions of a fluid-filled body cavity are:

·        The fluid cushions the organs thus preventing injury.

·        Internal organs can grow and move independently of the outer body wall.

·        Serves as a hydrostatic skeleton in soft bodied coelomates such as earthworms.

 

            In addition to the presence of a body cavity, acoelomates differ from     pseudocoelomates and eucoelmates by not having a blood vascular system.

 

D.                The Protostome-Deuterostome Split

           

            Distinguished by differences in development, the coelomate phyla can be            divided into two distinct evolutionary lines: protostomes and       deuterostomes

           

            Developmental differences between protostomes and deuterostomes     include: cleavage patterns, fate of the blastopore, and coelom formation.

 

1.      Cleavage Patterns

            Most protostomes undergo spiral cleavage and determinate       cleavage during their development.

 

            Spiral Cleavage : Cleavage in which the planes of cell division are          diagonal to the vertical axis of the embryo.

           

            Determinate cleavage: Cleavage in which each embryonic cell    retains the capacity to develop into a complete embrya if isolated       from other cells. This type of cleavage in the human zygote results          in identical twins.

 

2.      Blastopore Fate

            Blastopore: The first opening of the archenterons which forms    during gastrulation.

           

·        In Protostomes, the blastopore forms the mouth.

·        In Deuterostomes, the blastopore forms the anus.

 

3.      Coelom Formation

                       

                                    Schizocoelous: Descriptive term for coelom development during                                                which, as the archenterons forms, the coelom begins to split within                                 the solid mesodermal mass; coelom formation found in                                                         Protostomes.

                                   

                                    Enterocoelous: Coelom development during which the mesoderm                                              arises as lateral outpocketing of the archentron with hollows that                                                become the coelomic cavities; coelom formation found in                                                     Deuterostomes.

                                   

           

 

 

 

III.             Sponges are sessile animals lacking true tissues

 

            The sponges, in the phylum Porifera, are the only members of the           subkingdom Parazoa due to their unique development and simple             anatomy.

 

·        Approximately 9,000 species, mostly marine with only about 100 in fresh water.

·        Lack true tissue and organs, and contain only two layers of loosely associated unspecialized cells.

·        No nerves or muscles, but individual cells detect and react to environmental changes.

·        Size ranges from 1 cm 2 m.

·        All are suspension-feeders

·        Possibly evolved form colonial choanaflagellates.

 

 

Parts of the Sponges:

 

            Spongocoel: Central cavity of sponge.

            Osculum: Larger excurrent opening of the spongocoel.

            Epidermis: Single layer of flattened cells which forms outer surface of the            sponges.

            Porocyte: cell which forms pores; possess a hollow channel throughthe center which extends from the outer surface to spongocoel.

            Choanocyte: Collar cell, majority of cells which line the spongocoel;                   possess a flagellum which is ringed by a collar of finger-like projections.       Flagellar movements moves water and food parricles which are trapped on        the collar and later phagocytized.

            Mesohyl: The gelatinous layer located between the two laters of the       sponge body wall.

            Amoebocytes: Wandering pseudopod bearing cells in the mesohyl;                    function in food uptake from choanocytes, food digestion , nutrient    distribution to other cells, formation of skeletal fibers, gamete formation.

            Spicule: Sharp, calcium carbonate or silica structures in the mesohyl       which form the skeletal fibers of many sponges.

            Spongin: Flexible, proteinaceous skeletal fibers in the mesohyl of some   sponges.

 

Sponges are hermaphroditic by usually cross-fertilize.

·        Eggs and sperm form in the mesohyl from differentiated amoebocytes or choanocytes.

·        Eggs remain in the mesohyl.

·        Sperm are released into excurrent flow of the spongocoel and are then drawn in with incurrent flow of another sponge.

·        Sperm penetrate into mesohyl and fertilize the eggs.

·        The zygote develops into a flagellated larva which is released into the spongocoel and escapes with the excurrent water through the osculum.

·        Surviving larvae settle on the substratum and develop. In most cases the larva turns inside-out during metamorphosis, moving the flagellated cells to the inside.

 

            Sponges possess extensive regeneration abilities for repair and asexual   reproduction.

 

IV.              Cnidarians and Ctenophores are radiate, diploblasic animals with gastrocascular           cavities.

           

A.                 Phylum Cnidaria

           

            There are more than 10,000 species in the phylum Cnidaria, most          of which are marine.

            The phylum contains hydras, jellyfish, sea anemones and coral   animals.

 

            Some characheristics of cnidarians include:

·        Radial symmetry

·        Diploblastic.

·        Simple, sac-like body.

·        Gastrovascular cavity present: a central digestive cavity with only one opening (functions as mouth and anus.)

 

 

                        There are two possible cnidarian body plans: sessile polyp and motile,                           floating medusa. Some species of cnidarians exist only as polyp, some                             only as medusa, and others are dimorphic.

                       

                        Polyp: Cylindrical form which adheres to the substratum by the aboral end                     of the body stalk and extends tentacles around the oral end to contact prey.

           

                        Medusa: Flattened, oral opening down, bell-shaped form. Moves freely in                                 water by passive drifting and weak bell contractions. Tentacles dangle                              from the oral surface which points downwards.

 

           

                        Cnidarians are carnivorous.

·        Tentacles around the mouth/anus capture prey animals and push them through the mouth/anus into the gastrovascular cavity.

·        Digestion begins in the gastrovascular cavity with the undigested remains being expelled through the mouth/anus.

·        Tentacles are armed with stinging cells, called snidocytes – after which the Snidaria are named.

 

                        Cnidocytes: Specialized cells of cnidarian epidermis that comatin                                                nematocysts, capsule-like structure with an eversible stringer; used in                             defense and capture of prey.

 

                        The simplest forms of muscles and nerves occur in the phylum cnidaria.

           

·        Epidermal and gastrodermal cells have bundles of microfilaments arranged into contractile fibers.

            à The gastrovascualar caity, when filled with water, acts as a hydrostatic          skeleton against which the contractile fibers can work to change the           animal’s shape.

·        A simple nerve net coordinates movement; no brain is present.

            à The nerve net is associated with simple sensory receptors radially     distributed on the body. This permits stimuli to be detected and responded                 to from all directions.

 

There are three major classes of cnidarians:

 

1.      Class Hydrozoa

 

                        Most alternate polyp and medusa forms in the life cycle although the                              polyp is the dominant stage. Some are colonial while others are solitary.

                        Hydra is unique in that only the polyp stage is present.

·        They usually reproduce asexually by budding; however, in unfavorable conditions they reproduce sexually. In this case a resisting zygote is formed and remains dormant until environmental conditions improve.

 

 

2.      Class Scyphozoa

 

                        The planktonic medusa (jellyfish) is the most prominent stage of the life                          cycle.

·        Coastal species usually pass through a small polyp stage during the life cycle.

·        Open ocean species have eliminated the polyp entirely.

 

           

3.      Class Anthozoa

 

                        This class contains sea anemones and coral animals which only occur as                                     polyp.

           

                        Coral animals may be solitary or colonial and secrete external skeletons of                     calcium carbonate.

 

·        Each polyp generation builds on the skeletal remains of earlier generations. In this way, coral reefs are formed.

·        Coral is the rock- like external skeletons

 

 

 

B.         Phylum Ctenophora

           

            This phylum contains the comb jellies. There are about 100 species, all of which             are marine.

 

            Some characteristics of Ctenophorans include:

·        A resemblance to the medusa of Cnidarians in that the body of most is spherical or ovoid; a few are elongated and ribbon-like.

·        Transparent body, 1-10 cm in diameter (spherical/ovoid forms) or up to 1 m long (ribbon-like forms).

·        Possess eight rows of comblike plates composed of fused cilia which are used for locomotion.

·        One pair of long retractable tentacles are present and function in capturing food; cnidocytes have been found in only one species.

·        A sensory organ containing calcareous particles is present.

            à The particles settle to the low point of the organ which then acts as an           orientation cue.

            à Nerves extending from the sensory organ to the combs of cilia          coordinate movements.

 

 

V.                 Flatworms and other acoelomates are bilateral, triploblastic animals        lacking body cavities.

 

A.                 Phylum Platyhelminthes

 

                                    The phylum Platyhelminthes differs from the phylum Cnidaria in                                      those they:

 

·        Exhibit bilateral symmetry with moderate cephalizaion.

·        Are triploblastic (develop from three-layered embryos: ectoderm, endoderm and mesoderm.)

·        Possess several distinct organs, organ systems and true muscles.

 

                                    Although more advanced than cnidarians, two things point to the                                                early evolution of platyhelminthes in bilateria history.

                                   

·        A gastrocascular cavity is present.

·        They have an acoelomate body plan.

 

                                    There are more than 20,000 species of Platyhelminthes which are                                              divided into four classes:

                                   

·        Class Turbellaria

·        Classes Trematoda and Monogenea.

·        Class Cestoda.

 

 

 

                        1. Class Turbellaria

 

                                    Mostly free-living, marine species; a few species are found                                                        in freshwater and moist terrestrial habitats.

                       

                                    Planarians are familiar and common freshwater forms

·        Carnivorous, feed on small animals and carrion.

·        Lack specialized organs for gas exchange or circulation.

            à Gas exchange is by diffusion (flattened body form places al cells close           to water).

            à Highly ramified gastrovascular cavity distributes food throughout the             body.

·        Flame cell excretory apparatus present which functions primarily to maintain osmotic balance of the animal.

            à Nitrogenous waste (ammonia) diffuses directly from cells to the water.

·        Move by using cilia on the ventral dermis to glide along a film of mucus. Muscular contractions produce undulations which allow some to swim.

·        On the head are a pair of eyespots which detect light and a pair of lateral auricles that are olfactory sensors.

            à Possess a rudimentary brain which is capable of simple learning.

·        Reproduce either asexually or sexually.

            à Asexual is by generation: mid-body constriction separates the parent                         into two halves, each or which regenerates the missing portion.

                      

 

                       2. Classes Trematoda and Monogenea

 

                       All members of these two classes are parasitic.

 

                       Flukes are members of the class Trematoda.

·        Suckers are usually present for attaching to host internal organs.

·        Primary organ system is the reproductive system; a majority are hermaphroditic.

·        Life cycles include alternations of sexual and asexual stages with asexual development taking place in an intermediate host.

           à Larvae produce by asexual development infect the final hosts where              maturation and sexual reproduction occurs.

·        Schistosoma spp. (blood flukes) infect 200 million people worldwide.

 

Members of the class Monogenea are mostly external parasites of fish.

·        Structures with lare and small hooks are used for attaching to the host animal.

·        All are hermaphroditic and reproduce sexually.

 

3. Class Cestoda

                      

                       Adult tapeworms parasitize the digestive system of vertebrates.

·        Possess a scolex (head) which may be armed with suckers and/or hooks that help maintain position by attaching to the intestinal lining.

·        Posterior to the scolex  is a long ribbon of units called proglottids.

           à A proglottid is filled with reproductive organs.

*    No digestive system is present.

 

           The life cycle of a tapeworm includes an intermediate host.

·        Mature proglottids filled with eggs are released from the posterior end of the worm and pass from the body with feces.

·        Eggs are eaten by an intermediate host and a larvae  develops, usually in muscle tissue.

 

                       à The final host becomes infected when it eats an intermediate host       containing larvae.

·        Humans can become infected with some species of tapeworms by eating poorly cooked beef or pork containing larvae.

 

 

B. Phylum Nemertea

 

           There are about 900 species, most are marine with a few in fresh water and                   damp soil.

 

           The phylum Neremertea contains the proboscis worms.

·        Size from 1 mm to more than 30 m.

·        Some active swimmers, others burrow is sand.

·        Possess a long, retractable hollow tube (proboscis) which is used to probe the environment, capture prey, and as defense against predators.

·        Excretory, sensory and nervous systems are similar to planarians.

·        Acoelomate.

 

 

There are some important differences between the Nemertea and Platyhelminthes.

 

·        Nemertea possess a simple circulatory system which consists of vessles through which blood flows. Some species have red blood cells containing a form of hemoglobin which transports oxygen. No heart is present, but body muscle contractions move the blood through vessels.

·        Nemertea possess a complete digestive system with a mouth and an anus.

 

                      

                       The phylogenetic position of the Nemertea is uncertain.

·        The body is structurally acoelomate, but the fluid-filled proboscis sac is considered a true coelom by some researchers.

·        A simple blood vascular system and a complete digestive system are characteristics shared with more advanced phyla.

 

 

VI.           Rotifers, nematodes, and other pseudocoelomates have complete digestive tracts an blood vascular systems

 

           The pseudocoelomate body plan probably arose independently several   times. Pseudocolomates are thought to be more closely related to     protostomes than deuterostomes, but more study of their true association is            required.

 

A.                 Phylum Rotifera

           

There are approximately 1,800 species of rotifers. They are small, mainly freshwater organisms, although some are marine and others are found in damp soil.

 

·        Size ranges from 0.05-2.0 mm.

·        Pseudocolomate with the pseudocolomatic fluid serving as a hydrostatic skeleton and as a medium which transports nutrients and wastes when the body moves.

·        Complete digestive system is present,

           à Bear a crown of cilia that draws a vortex of water into the mouth.

           à Posterior to the mouth, a jaw-like organ grinds the microscopic food organisms suspended in water.

 

Reproduction is rotifers may be by parthenogenesis or sexual.

·        Some species consist only of females with new females developing by parthenogenesis from unfertilized eggs.

·        Other species produce two types of eggs, one that develops into females, the other into degenerate males.

           à Males produce sperm that fertilize eggs which develop into resistant   zygotes that survive desiccation.

           à When conditions improve, the zygote break dormancy and develop into         a new female generation that reproduces by parthenogenesis until      unfavorable conditions return.

·        Rotifers have no regeneration or repair abilities.

 

Rotifers contain a certain and consistent number of cells as adults. The zygotes undergo a specific number of divisions and the adult contains a fixed number of cells.

 

B. Phylum Nematoda

 

            There are more than 80,000 species of roundworms ranging in sizes from less    than 1.0 mm to more than 1 m.

 

·        Bodies are cylindrical with tapered ends.

·        Very numerous in both species and individuals.

·        Found in fresh water, marine, moist soil, tissues of plants, and tissues and body fluids of animals.

·        A complete digestive tract is present and nutrients are transported through the body in the pseudocoelomic fluids.

·        A tough transparent cuticle forms the outer boy covering.

·        Longitudinal muscles are present and provide for whip-like movements

·        Dioecious with females larger than males.

·        Sexual reproduction only, with internal fertilizations

·        Female may produce 100,000 or more resistant eggs per day.

·        Like rotifers, nematodes have a fixed number of cells as adults.

Nematodes fill various roles in the community.

·        Free-living forms are important in decomposition and nutrient cycling.

·        Plant parasitic forms are important agricultural pests.

·        Animal parasitic forms can be hazardous to health (Trichinella spiralis in humans via poorly cooked pork.)

 

 

VII. Mollusks and annelids are among the major variations on the protostome body plan.

 

            The protostome lineage of coelomate animals gave rise to many phyla. In many,             the coelom functions as a hydrostatic skeletom.

           

  1. Phylum Mollusca

 

      There are more than 50,000 species of snails, slugs, oysters, clams, octopus, and           squids.

 

·        Mollusks are mainly marine, though some inhabit fresh water and many smails and slugs are terrestrial.

 

Mollusks are soft-bodied, but most are protected by a hard calcium carbonate shell.

 

·        Squids and octopuses have reduced, internalized shells or no shell.

 

The molluscan body consists of three primary parts: muscular foot for locomotion, a visceral mass containing most of the internal organs, and a mantle which is heavy fold of tissue that surrounds the visceral mass and secretes the shell.

 

A radula is present in many functions as a rasping tongue to scrap food from surfaces.

 

Some species are monoecious while most are dioecious.

·        Gonads are located in the visceral mass.

 

Some zoologists believe the mollusks evolved from annelid-like ancestors ( although true segmentation is absent) while others believe that mollusks arose earlier in the protostomes lineages before segmentation evolved.

 

1. Class Polyplacophora

 

    The Class Polyplacophora contains the marine species known as chitons.

·        They have an oval shape with the shell divided into eight dorsal plates.

·        Cling to rocks along the shore at low tide using the foot as a suction cup to grip the rock.

·        Move slowly on its, broad muscular foot when submerged.

·        A radula is used to cut and ingest (‘graze’) algae.

 

2. Class Gastropoda

   

    The Class Gastropoda contains the snails and slugs.

·        Largest molluscan class with more than 40,000 species.

·        Mostly marine but many species are freshwater or terrestrial.

·        Torsion during embryonic development is a distinctive characteristic:

            à Uneven growth in the visceral mass causes the visceral mass to rotate           180 degrees, placing the anus above the head in adults.

·        Body protected by a shell (absent in slugs and nudibranchs) which may be conical or flattened.

·        Many species have distinct heads with eyes at the tips of tentacles.

·        Most gastropods are herbivorous, using the radula to graze on plant material; several groups are predatory and possess modified radulae.

·        Most aquatic gastropods exchange gases via gills; terrestrial forms have lost the gills and utilize a vascularized lining of the mantle cavity for gas exchange.

 

3.Class Bivalvia

           

      This class contains the clams, oysters, mussels and scallops.

 

       Possess a shell divided into two halves.

·        The shell halves are hinged at the mid-dorsal line and are drawn together by two adductor muscles to protect the animal.

·        Bivalves may extend the foot for motility or anchorage when the shell is open.

·        The mantle cavity (between shells) contains gills which function in gas exchange and feeding.

·        Most are suspension feeders that trap small food particles in the mucus coating of the gills and then use cilia to move the particles to the mouth.

            à Water enters the mantle cavity through an incurrent siphon, passes over        the gills, and then exits through the excurrent siphon.

            à No radula or distinct head is present.

 

Bivalves lead sedentary lives. They use the foot as an anchor in sand or mud. Sessile mussels secrete threads that anchor them to rocks, docks, or other hard surfaces.

 

Scallops can propel themselves along the sea floor by flapping their shells.

 

 

 

                      

 

4. Class Cephalopoda

 

           This class contains the squids and octopuses.

 

           Cephalopods are agile carnivores.

·        Use beak like jaws to crush their prey.

·        The mouth is at the center of several long tentacles

 

           A mantle covers the visceral mass, but the shell is either reduced and       internal (squids) or totally absent (octopus)

 

·        The chambered nautilus is the only shelled cephalopod alive today.

·        Squids swim backwards in open water by drawing water into the mantle cavity, and then firing a jet stream of water through the excurrent siphon which points anteriorly.

           à Directional changes can be made by pointing the siphon in different    directions.

           à Most squid are less than 75 cm long but the giant squid may reach 17 m         and weigh 2 tons.

·        Octopuses usually don’t swim in open water, but move along the sea floor in search of food.

 

 

           Cephalopods are the only mollusks with a closed circulatory system in    which the blood is always contained in vessels.

          

           Cephalopods have well developed nervous systems with complex brains             capable of learning. They also have well developed sense organs.

 

           The cephalopods ancestors were probably shelled, carnivorous forms - the         ammonites.

·        These cephalopods were the dominant invertebrate predators in the oceans until they became extinct at the end of the Cretaceous.

 

 

B. The Lophophorate Animals

 

           The Lophophorate animals contain three phyla: Phoronida, Bryozoa and             Brachiopoda.

          

·        These three phyla are grouped together due to presence of a lophophore.

 

                       Lophophore = Horseshoe- shaped or circular fold of the body wall bearing       ciliated tentacles that surround the mouth at the anterior end of the animal.

·        Cilia direct water toward the mouth between the tentacles which trap food particles for these suspension feeders.

·        The presence of a lophophore in all three groups suggests a relationship among these phyla.

 

The three phyla also possess a U-shaped tract ( the anus lies outside of the        tentacles)  and have no distinct head -  both adaptations for a sessile existence.

 

Lophophorates are difficult to assign as protostomes or deuterostomes.

·        Their embryonic development more closely resembles deuterstomes; however, in the phoronida, the blastopore develops into the adult mouth.

·        Molecular systematic places the lophophorate phyla closer to the protostomes than the deuterostomes.

 

1. Phylum Phoronida

 

This phylum contains about 15 species of tube-dwelling marine worms.

·        Length from 1 mm to 50 cm.

·        Phoronids live buried in sand in chitinous tubes with the lophophore extended from the rube when feeding.

 

2. Phylum Bryozoa

 

This phylum contains the moss animals. There are about 5,000 species which are mostly marine and are widespread.

 

Bryozoans are small, colonial forms

·        In most, the colony is enclosed within a hard exoskeleton and the lophophores are extended through pores when feeding.

·        Some are important reef builders.

 

3. Phylum Brachiopoda

 

This phylum contains the lamp shells. There are approximately 330 extant species, all marine.

·        More than 30,000 fossil species of the Paleozoic and Mesozoic have been identified.

 

The body of brachiopod is enclosed by dorsal and ventral shell halves.

·        Attach to the substratum by a stalk.

·        Open the shell slightly to allow water to flow through the lophophore.

 

 C. Phylum Annelida

           

            The presence of a true coelom and segmentation are two important evolutionary            advances present in the annelids.

 

·        The coelom serves as a hydrostatic skeleton, permits development of complex organ systems, protects internal structure, and permits the internal organs to function separately from the body wall muscles.

·        Segmentation also provided for the specialization of different body regions.

 

There are more than 15,000 species of annelids.

·        They have segmented bodies and range in size from less than 1 mm to 3 m.

·        There are marine, freshwater, and terrestrial ( in damp soil) annelids.

 

Annelids have a coelom partitioned by septa. The digestive tract, longitudinal blood vessels, and nerve cords penetrate the septa and extend the length of the animal.

 

The complete digestive system is divided into several parts, each specialized for a specific function in digestion:

 

Pharynx à Esophagus à Crop à Gizzard à Intestine

 

Annelids have a closed circulatory system.

·        Hemoglobin is present in blood cells.

·        Dorsal and ventral longitudinal vessels are connected by segmental pairs of vessels.

·        Five pairs of hearts circle the esophagus.

·        Numerous tiny vessels in the skin permit gas exchange is across the body surface.

 

An excretory system of paired metanephridia is found in each segment; each metanephridium has a nephrostome (which removes wastes from the coelomic fluid and blood) and exits the body through an exterior pore.

 

The annelid nervous system is composed of a pair of cerebral ganglia lying above and anterior to the pharynx.

·        A nerve ring around the pharynx connects these ganglia to a subpharyngeal ganglion, from which a pair of fused nerve cords run posteriorly.

·        Along the ventral nerve cords are fused segmental ganglia.

 

Annelids are hermaphroditic but cross-fertilize during sexual reproduction.

·        Two earthworms exchange sperm and store it temporarily.

·        A special organ, the clitellum, secretes a mucous cocoon which slides along the worm, picking up its eggs and then the stored sperm.

·        The cocoon slips off the worm into the soil and protects the embryos while they develop.

·        Asexual reproduction occurs in some species by fragmentation followed by regeneration.

 

Movement involves coordination longitudinal and circular muscles in each segment with the fluid-filled coelom functioning as a hydrostatic skeleton.

·        Circular muscle contraction makes each segment thinner and longer; longitudinal muscles contraction makes the segment shorter and thicker.

·        Waves of alternating contractions pass down the body.

·        Most aquatic annelids are bottom-dwellers that burrow, although some swim in pursuit of food.

 

1. Class Oligochaeta

 

            This class contains earthworms and a variety of aquatic species.

           

            Earthworms ingest soil, extract nutrients in the digestive system and deposit        undigested material (mixed with mucus from the digestive tract) as casts through             the anus.

 

·        Important to farmers as they till the soil and casting improve soil texture.

·        Charles Darwin estimated that 1 acre of British farmland had about 50,000 earthworms that produce 18 tons of castings per year.

 

2. Class Polychaeta

 

            This class contains mostly marine species.

           

            A few drift and swim in the plankton, some crawl along the sea floor, and many             live in tubes they construct by mixing sand and shell bits with mucus.

·        Tube-dwellers include the fanworms that feed by trapping suspended food particles in their feathery filters which are extended from the tubes.

 

Each segment has a pair of parapodia which are highly vascularized paddle-like structures that function in gas exchange and locomotion.

·        Traction for locomotion is proved by several chitinous setae present on each parapodium.

 

           

3. Class Hirudinea

 

            This class contains the leeches.

·        A majority of species are freshwater by some are terrestrial in moist vegetation.

·        Many are carnivorous and feed on small invertebrates while some attach temporarily to animals to feed on blood.

·        Size ranges from 1 -30 cm in length.

 

            Some blood-feeding forms have a pair of blade-like jaws that slit the host’s skin            while others secrete enzymes that digest a hole in the skin.

·        An anesthetic is secreted by the leech to prevent detection of the incision by the host.

·        Leeches also secrete  hirudin which prevents blood coagulation during feeding.

·        Leeches may ingest up to 10 times their weight in blood at a single meal and may not feed again for several months.

·        Leeches are currently used to treat bruised tissues and for stimulating circulation of blood to fingers and tow reattached after being severed in accidents.

 

 

 

 

VII. The protostome phylum Arthropoda is the most successful group of animals ever to           live.

 

This phylum is the largest phylum of animals with approximately 1 million described species.

·        Arthopods are the most successful phylum based on species diversity, distribution, and numbers of individuals.

 

A. General Characteristics of Arthropods

 

            The success and great diversity of arthropods is related to their segmentation,    jointed appendages, and hard exoskeleton.

           

            The segmentation in this group is much more advanced than that found in           annelids.

           

·        In the arthropods, different segments of the body and their associated appendages have become specialized to perform specialized functions.

·        Jointed appendages are modified for walking, feeding sensory reception, copulation and defense.

 

            The arthropod body is completely covered by the cuticle, and exoskeleton         (external skeleton) constructed of layers of protein and chitin.

           

·        The cuticle is thin and flexible in some locations (joints) and thick and hard in others.

·        The exoskeleton provides protections and points of attactment for muscles that move appendages.

·        The exoskeleton is also relatively impermeable to water.

·        The old exoskeleton must be shed for an arthropod to grow (molting) and a new one secreted.

 

Arthropods show extensive cephalization with many sensory structures clustered at the anterior end. Well developed sense organs including eyes, olfactory receptors, and tactile receptors are present.

An open circulatory system containing hemolymph is present.

·        Hemolymph leaves the heart through short arteries and passes into the sinuses which surround the tissue and organs.

·        The hemolymph reenters the heart through pores equipped with valves.

·        The blood sinuses comprise the hemocoel. Thoughthe hemocoel is the main body cavity, it is not part of the coelom.

·        The true coelom is reduced in adult arthrops.

 

 

Gas exchange structures are varied and include:

·        feathery gills in aquatic species.

·        Tracheal systems in insects.

·        Book-lungs in other terrestrial forms. (e.g. spiders)

 

 

B. Arthropod Phylogeny and Classification

 

            Arthropods are segmented protostomes which probably evolved from annelids or          a segmented protostome common ancestor.

           

·        Early arthropods may have resembled onychophorans which have unjointed appendages.

            à However, many fossils of jointed-legged animals resembles segmented          worm support the evolutionary link between the Annelida and Arthropoda.

            à Such comparisons also indicate that annelids and arthropods are not             closely related.

·        Parapodia may have been forerunners of appendages.

·        Some systematists suggest that comparisons of ribosomal RNA and other macro molecules indicate that onychophorans are arthropods and not transitional forms.

·        This evidence presents an alternative hyphothesis that segmentation evolved independently in annelids and arthropods.

            à Thus, the most recent common ancestor of these two common          ancestors of these two phyla would have been an unsegmented        protostome.

 

Although the origin of arthropods is unclear, most zoologists agree that four main evolutionary lines can be identified in the arthropods. Their divergence is represented by the subphyla: Trilobitmorpha (extinct trilobites), Cheliceriformes, Uniramia, and Crustacea.

 

C. Trilobites (subphylum trilobitomorpha)

 

            Early arthropods called trilobites were very numerous but became extinct           approximately 250 million years ago.

·        Trilobites had extensive segmentation, but little appendage specialization.

·        As evolution continued, the segments tended to fuse and appendages became specialized for a variety of functions.

 

D. Spiders and Other Chelicerates (Sub phylum Cheliceriformes)

 

            Other early arthropods included chelicerates such as the eurypteris (sea scorpions)        which were predaceous and up to 3 m in length.

 

·        The chelicerate body is divided into an anterior cephalothorax and a posterior abdomen.

·        Their appendages were more specialized than those of trilobites, with the most anterior ones being either pincers or fangs.

·        Chelicerates are named for their feeding appendages, the chelicerae.

            à Only one marine chelicerate remains, the horse – shoe crab.

            à Other chelicerates include terrestrial spiders, scorpions, ticks and mites         of the Class Arachnida which lack antennae and have simple eyes.

 

Members of the Class Arachnida possess a cephalothorax with six pairs of appendages: chelicerae, pedipalps (used in sensing and feeding), and four pairs of walking legs.

 

In spiders,

·        Fang-like chelicerae, equipped with poison glands, are used to attack prey.

·        Chelicerae and pedipalps masticate the prey while digestive juices are added to the tissue.

            à This softens the food and the spider sucks up the liquid.

·        Gas exchange is by book-lungs (stacked plates in an internal chamber).

            à The structure of the book-lung provides an extensive surface area for            exchange.

 

Spiders weave silken webs to capture prey.

·        The proteinaceous silk is produced as liquid by abdominal glands and spun into fibers by spinnerets. The fiber hardens on contact with air.

·        Web production is apparently an inherited complex behavior.

·        Silk fibers are also used for escape, egg covers, and wrapped around food presented to females during courtship.

 

 

E. Comparison of Chelicerates to Uniramians and Crustaceans.

 

            Another major line of arthropod evolution gave rise to the subphylum Uniramia and        Crustacea. These groups are characterized by:

·        Jaw-like mandibles instead of claw-like chelicerae.

·        One or two pairs of sensory antennae and a pair of compound eyes.

 

            The subphylum Uniramia includes millipedes, centipedes and insects:

·        All possess one pair of antennae and unbranched (uniramous) appendages.

·        Are believed to have evolved on the land, aided by the presence of the exoskeleton first evolved in marine forms.

 

The subphylum Crustacea includes the crustaceans, which:

·        Have two pairs of antennae and branched (biramous) appendages.

·        Are primarily aquatic and believed to have evolved in the ocean.

 

Chelicerates and uniramians moved onto land during the Devonian period.

·        They were able to move into terrestrial habitats partly due to the presence of the exoskeleton.

            à The main functions of the exoskeleton in the seas, where it evolved, were protection and muscle attachment.

            à In terrestrial habitats, the exoskeleton provided support and prevented          desiccation.

·        Fossilized burrows of millipede-like arthropods are the oldest (about 450 million year) evidence of terrestrial animals.

            à The presence of a protective cuticle reduces water loss.

            à A firm exoskeleton also provided support without the buoyancy of               water.

 

 

 

F. Insects and Other Uniramians

 

            This subphylum includes three classes: Diplopoda, Chilopoda, and Insecta.

 

            The class Diplopoda includes the millipedes.

·        Worm-like with a large number of walking legs (2 pair per segment)

·        Eat decaying leaves and other plant matter.

·        Probably among the earliest land animals.

 

The class Chilopoda includes the centipedes.

·        They are carnivorous

·        One pair of antennae and three pairs of appendages modified as mouthparts (including mandibles) are located on the head.

·        Each trunk segment has one pair of walking legs.

·        Poison claws on the most anterior trunk segment are used to paralyze prey and for defense.

 

The class Insecta has greater species diversity than all other forms of life combined.

·        There are about 26 orders of insects.

·        They inhabit terrestrial and freshwater environments, but only a few marine forms exist.

 

Entomology = the study of insects.

 

The oldest insect fossils are from the Devonian period (about 400 million years ago), and an increase in insect diversity can be attributed to:

·        The evolution of flight during the Carboniferous and Permian.

·        The evolution of specialized mouth parts for feeding on gymnosperms and other Carboniferous plants.

            à The fossil record holds examples of a diverse array of specialized     mouth parts.

 

A second major radiation of insects which occurred during the Crateous period was one thought to have paralleled radiation of flowering plants.

·        Current research indicates the major diversification of insects preceded angiosperm radiation during the Cretaceous period (65 million years ago)

·        If this is true, insect diversity played a major role in angiosperm radiation, the reverse of the original hypothesis.

 

 

Flight is the key to the success of insects, enabling them to escape predators, find food and mates, and disperse more easily than non-flying forms.

·        One or two pairs of wings emerge from the dorsal side of the thorax in most species.

·        Wings are extensions of the cuticle and not modified appendages.

·        Wings may have first evolved to help absorb heat, and then developed further for flight.

·        Other views suggest wings may have initially served for gliding, as gills in aquatic forms, or even as structures for swimming.

 

 

            Dragonflies were among the first to fly and have two coordinated pairs of wings.

            Modifications are found in groups which evolved later.

·        Bees and wasp hook their wings together (act as one pair)

·        Butterflies have overlapping anterior and posterior wings.

·        Beetles have anterior wings modified to cover and protect the posterior (flying) wings.

 

            There are three main regions to the anatomy of and insect: Head, Thorax, and    Abdomen.

·        Head segments are fused while segmentation is apparent on the thorax and abdomen.

·        The head has: one pair of antennae, one pair of compound eyes, and several pairs of appendages modified as feeding mouthparts ( for chewing of lapping, piercing sucking – depending on the insect.)

·        The thorax has three pairs of walking legs and the wings.

 

            Insects have several complex internal organs systems.

·        Complete digestive system with specialized regions.

·        Open circulatory system with hemolymph.

·        Excretory organs are the Malpighian tubules which are outpocketings of the gut.

·        Gas exchange is by a tracheal system which opens to the outside via spiracles that can open or close to regulate air and limit water loss.

·        Nervous system is composed of a pair of ventral nerve cords (with several segmental ganglia) which meet in the head where the anterior ganglia are fused into a dorsal brain close to the sense organs.

            à Insects show complex behavior which is apparently inherited            (e.g. social behavior of bees and ants.)

 

            Many insects undergo metamorphosis during their development.

                       

            Incomplete metamorphosis = a type of development during which young        resemble adults but are similar and have different body proportions.

·        For example, in grasshoppers a series of molts occur with each stage looking more like an adult until full size is reached.

 

            Complete metamorphosis= a type of development characterized by larval stages        which are very different in appearance from adults.

·        Larvas eat and grow before becoming adults.

·        Adults find mates and reproduce with the females laying eggs on the appropriate food source for the larval forms.

 

 

 

            Insects are dioecious and usually reproduce sexually with internal fertilization.

·        Sperm are deposited directly into the female’s vagina during copulation.

·        Inside the female, sperm are stored in the spermatheca.

·        Most insects produce eggs although some flies are viviparous.

·        Many insects mate only one in a life time with stored sperm capable of fertilizing many batches of eggs.

 

 

            Insects impact terrestrial organisms in a number of ways. They:

·        Compete for food.

·        Serve as disease vectors.

·        Pollinate many crops and orchards.

 

G. Crustaceans

           

            There are more than 40,000 species of crustaceans in marine and fresh waters.

 

            The crustaceans have extensive specialization of their appendages.

·        Two pairs of antennae, three or more pairs of mouthparts including mandibles, walking legs on the thorax, appendages are present on the abdomen.

·        Lost appendages can be regenerated.

 

            Characteristics of their physiology include:

·        Gas exchange may take place across thin areas of the cuticle (small forms) or by gills (large forms).

·        An open circulatory system in present with hemolymph.

·        Nitrogenous waste is excreted by diffusion across thin areas of the cuticle.

·        Salt balance of the hemolymph is regulated by a pair of        specialized antennal or maxillary glands.

 

                                   

            Most are dioecious and some males have specialized pair of appendages to       transfer sperm to the female’s reproductive pore during copulation.

·        Most aquatic crustaceans have at least one swimming larval stage.

 

            The decapods are relatively large crustaceans that have carapace ( calcium        carbonate hardened exoskeleton over the cephalothorax). Examples:

·        Freshwater crayfish

·        Marine lobster, crab, and shrimp

·        Tropical land crabs.

 

            The isopods are mostly small marine crustaceans but include terrestrial sow bugs            and pill bugs.

·        Terrestrial forms live in moist soil and damp areas.

 

            Copepods are numerous small marine and firewater planktonic crustaceans

·        The larva of the larger crustaceans may also be plantonic.

 

            Barnables are sessile crustaceans with parts of their cuticles hardened into shells             by calcium carbonate.

·        Barnacles feed by directing suspended particles towards the mouth with specialized appendages.

 

 

 

IX. The deuterostome lineage includes echinodermats and chordates.

 

            The deuterostomes, while a very diverse group, share characteristics which        indicate their association: radial cleavage, enterocoelous coelom formation, and             the blastorpore forms the anus.

 

A. Phylum Echinodermata

           

            Most echinoderms are sessile or sedentary marine forms with radial symmetry as           adults.

·        Internal and external parts radiate from the center, often as five spokes.

·        A thin skin covers a hard calcareous plate-like exoskeleton.

·        Most have bumps and spines which serve various functions.

 

            A unique feature of Echinoderms is the water vascular system.

 

            Water vascular system = a network of hydraulic canals which branch into      extensions called tube feet that function for locomotion, feeding and gas            exchange.

 

            Echinoderms are dioecious with sexual reproduction and external fertilization.

·        Bilaterally symmetrical larvae metamorphose into radial adults.

·        Early embryonic development exhibits the characteristics of deuterostomes.

 

            There are about 7,000 species of echinoderms, all of which are marine. The six recognized classes are:

 

1. Class Asteroidea

           

            This class includes the sea stars which have five or more arms extending                                     form a central disc.

·        Tube feet on the undersurface of the arms are extended by fluid forced into them by contraction of their ampulla.

·        Suction cups at the end of each tube foot attach to the substratum and muscles in the tube foot wall contract and shorten the foot.

 

            Coordination of extension, attaching, contraction and release allow slow            movement and attachment to prey.

·        Prey are obtained by attaching tube feet to the shells of clams and oysters; the arms of the sea star wrap around the prey and hold tightly using the tube feet.

·        The muscles of the mollusk fatigue and the shell is pulled open.

·        The sea stars evert their stomachs between the shells halves and secrete digestive juices onto the soft tissue of the mollusk.

 

Sea stars have a strong ability to regenerate, although a single arm cannot regenerate an entire body. Fishermen chopping up sea stars may actually increase their numbers.

 

2. Class Ophiuroidea

 

            This class contains the brittle stars which differ from sea stars in that they have:

·        Smaller central discs than sea stars.

·        Longer, more flexible arms than sea stars.

·        No suckers on their tube feet.

·        Locomotion is by serpentine lashing of flexible arms.

·        Varying feeding mechanisms.

 

3. Class Echinoidea

 

            This class contains the sea urchins and sand dollars.

·        Sea urchins are spherical in shape while sand dollars are flattened in the oralaboral axis.

 

            Echinoideans lack arms but have:

·        Five rows of feet present that provide slow movement.

·        Spines are pivoted to aid in locomotion.

 

            Echinoideans have a complex jaw-like structure present around the mouth which            is used for feeding on seaweeds and other food.

 

4. Class Crinoidea

           

            This class contains the sea lilies.

·        Most sea lilies are sessile, living attached to substratum by stalks

·        Motile sea lilies use their arms for a crawling form of locomotion as well as for feeding.

·        Arms circle the mouth (which points upward)  and are used in suspension feeding.

 

            Crinoids have exhibited a very conservative evolution.

·        Extant forms are very similar to fossilized forms from Ordovician period (500 million years ago.)

 

 

5. Class Holothuroidea

           

            This class contains the sea cucumbers which have little resemblance to other      echinoderms.

·        The lack spines

·        The hard endoskeleton is reduced.

·        The body is elongated in the oral-aboral axis.

 

            Species in the Holothuriodea do possess five rows of tube feet, a part of the      unique water vascular system.

·        Some tube feet around the mouth have developed into feeding tentacles.

 

6. Class Concentricycloida

 

            This class contains the sea daisies which are small (less than 1 cm), disc-shaped             marine animals.

·        Live in deep water

·        Don’t possess arms.

·        Tube feet are located around the disc margin

·        Possess a rudimentary digestive system or an absorptive velum on the oral surface.

·        Water vascular system consists of two concentric ring canals.

 

B. Phylum Chordata

 

            The Chordata diverged form a common deuterostome ancestor with echinoderms          at least 500 million years ago.

·        The two phyla are grouped together due to similarities in early embryonic development.

·        This phylum contains three subphyla: Urochordata, Cephalochordata and Vertebrata.

 

X. The Cambrian explosion produced all the major animal body plans.

 

            The animal kingdom probably originated from colonial protests related to           choanoflagellates.

 

            The diversification that produced many phyla occurred in a relatively short time on the geological scale. This evolutionary episode is called the Cambrian       Explosion.

 

            The Cambrian explosion encompassed a 5 to 10 million year time san at the       beginning of the Cambrian period (545 million years ago)

·        All the major animal body plans seen today evolved during this time.

·        New taxa appeared later but were variations on the basic plans already evolved. For example, mammals evolved about 220 million years ago but are only a variation of the chordate body plan which evolved during the Cambrian explosion.

 

            A much less diverse fauna preceded the Cambrian explosion.

·        This Precambrian fauna dated back to the Ediacaran period (700 million years ago).           

            à This period is name for Edicara Hills Of Australia where       Precambrain animals fossils were first discovered.

            à Fossils similar in age to these have since been discovered on             other continents.

·        Most Edicaran fossils appear to be cnidarians although bilaterial animals are also indiacated by fossilized burrows probably left by worms.

 

 

            The diversity of Cambrian animals is represented in three fossil beds:

·        The Burgess Shale in British Columbia is the best known.

·        A fossil bed in Greenland and one in the Yunnan region of China predate the Burgess Shale by 10 million years.

            à These beds indicate animal diversification occurred within 5 million years of the start of the Cambrian.

 

            Two contrasting interpretations of Burgess Shale fossils have been proposed:

·        The Cambrian explosion resulted in a large number of phyla which included the current phyla, many of which are now extinct.

            à During the mass extinction at the end of the Canbrian, only base        stock of 35 or so extant phyla survived.

·        The diversity of the Cambrian fossils represents ancient variations within the taxonomic boundaries of extant phyla.

            à As these fossils undergo continued study, many are classified            into extant phyla. This the number of exclusively Cambrian fossils is decreasing.

 

Several hypotheses about external factors have been proposed as explanations for the Cambrian explosion and the lack of subsequent major diversification.

·        The Cambrian explosion was an adaptive radiation resulting from the origin of the first animal.

            à These early animals diversified as they adapted to the various,           previously unoccupied, ecological niches.

·        Predator-prey relationships emerged and triggered diverse evolutionary adaptions.

            à Various kinds of shells and different forms of locomotion                 evolved as defense mechanisms against predation.

            à Predator also evolved new mechanisms to capture prey.

·        Major environmental change provided an opportunity for diversification during the Cambrian explosion.

            à The accumulation of atmospheric oxygen may have finally     reached a concentration to support the more active metabolism       needed for feeding and other activities by mobile animals.

 

 

Other hyptheses for the Cambrian explosion have emphasized internal changes in the organisms.

·        Origin of mesoderm may have stimulated diversification of the body plan.

·        Variation in genes that control pattern formation during animal development may have played a role in diversification.

            à Some of the genes that determine features such as segmentation        and placement of appendages and other structers are common to             diverse animals phyla.

            à Variation in expression of these genes during development    results in morphological differences that distinguish the phyl.

            à This same kind of variations in expression may have resulted in         the relatively rapid origin of diverse animal types during the Cambrian explosion.

            à The phyla, one developed, may have become locked into     developmental patterns that permitted subtle variation to allow         speciation and the origin of lower taxa, but prevented large scale             morphological evolution resulting in new phyla.

 

 

The hypotheses presented for external and internal factors are not mutually exclusive. A combination of factors may have combined to produce the Cambrian explosion.