CHAPTER SEVENTEEN: THE EVOLUTION OF ANIMALS All animals are either invertebrates or vertebrates. The vast majority of animal species (over 97%) are invertebrate species. All animals: are multicellular, are some sort of heterotrophs: free living, mutualistic, commensal or parasitic, are aerobic, have a body symmetry plan of some sort, such as Spherical Radial Bilateral exhibit cephalization, have a type of gut, either have or lack a coelom cavity and if they are more advanced they will have a pattern of body segmentation.

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SURVEY OF INVERTEBRATE ANIMALS

PHYLUM PORIFERA Have the most simplistic animal body design yet are extremely successful.

THE SPONGES

Lack any symmetry. They are called asymmetrical. Motile as juveniles, sessile as adults. Reproduce both asexually and sexually. Are marine filter feeders. Body support structure is called the spicule, which can be composed of protein, calcium, or silicon depending upon the species of sponge.

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PHYLUM CTENOPHORA Most are plankton.

COMB JELLIES

Are marine life forms. Have a simple nerve net: the first type of nervous system. Use only sexual reproduction. Use eight rows of cilia (combs) for locomotion. Saclike body structure. Comb jellies have a radial form of symmetry.

PHYLUM CNIDARIA These organisms are radially symmetrical.

HYDRA AND JELLYFISH

Most are marine forms. Have defensive structures called nematocysts that are produced by special cells named cnidocytes. Two different body plans: the polyp and the medusa. Have distinctive body tissues. Hydra, various jellyfishes, coral reef building organisms, sea anemones and the colony of polyps and medusae called the Portuguese Man o’ War are included in this group. Use sexual reproduction; produce a zygote that develops into a free-swimming planula larva. Usually settles and becomes sessile as adult.

PHYLUM PLATYHELMINTHES Bilaterally symmetrical body design.

FLATWORMS

These organisms are acoelomate and lack segmentation. Distinctive head region at one end of body (cephalization). Use sexual reproduction; many species are hermaphrodites. Includes free-living as well as parasitic forms. Habitats range from freshwater to marine to terrestrial. Three important classes within this phylum:

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CLASS TURBELLARIA (THE PLANARIANS)

Free-living flatworms, such as the Planarian.

CLASS TREMATODA (THE LIVER FLUKES)

Liver flukes, which parasitize animal livers.

CLASS CESTODA (THE TAPEWORMS)

Tapeworms with a head called a scolex and body segments called proglottids.

PHYLUM NEMATODA ROUNDWORMS These worms are bilaterally symmetrical and have cephalization. Many (but not all) are parasites. Include both free-living as well as parasitic forms. Free-living forms are marine, freshwater, or terrestrial. Parasitic forms start their life in an organisms gut and then migrate to a specific body structure while maturing to adult forms. EX: Trichina worms mature in muscles, Ascarid worms mature in lungs. These organisms have a pseudocoelom and lack segmentation. Examples of nematodes include: Heart worms

Affects both dogs and cats.

Pin worms

Affects children (primarily).

Trichina worms

Causes trichinosis (from pork).

Ascaris worms

Affects lungs of humans.

Schistosoma worms

Causes swimmer’s itch.

PHYLUM MOLLUSKA These soft bodies animals exhibit three distinctive body features:

MOLLUSKS

Head Foot Mantle with visceral organs Most mollusks use gills for respiration. Most adult forms have a full or partially reduced shell composed of calcium carbonate. Bilateral symmetry as juveniles, but they specialize to become radial as adults.

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Open circulatory system, except the cephalopod group. Cephalopods are advanced forms with a closed circulatory system (squid and octopus). Some forms (clams and chitons) contain a feeding organ called a radula that scrapes food off environmental surfaces. Mollusks are the second largest invertebrate group. The Arthropods (which includes insects) are the largest invertebrate group. Included in the group are snails, slugs, clams, nudibranchs, chitons, scallops, octopuses, cuttlefish and squids.

PHYLUM ANNELIDA ANNELIDS These organisms have bilateral symmetry, a true coelom, cephalization and body segmentation. Pretty darn specialized! This group includes free living as well as parasitic forms. CLASS OLIGOCHAETA (EARTH WORMS) CLASS POLYCHAEATA (MARINE WORMS) CLASS HIRUDINEA (LEECHES) Have a nerve cord, two types of muscles (circular and longitudinal) and a waste system that regulates body fluids (nephridia). Use a crop and gizzard in food processing.

Closed circulatory system with small circular ‘hearts’ (aortic arches).

PHYLUM ARTHROPODA ARTHROPODS This is the largest of all invertebrate groups in terms of total number of species with over 1,000,000 species. These coelomate animals have these special features:

Exoskeleton made of chitin. Segmentation with expanded body segments (choice of two segments: a cephalothorax and head or three segments: head, thorax and abdomen. Jointed appendages. Specialized respiratory organs and an open circulatory system. Division of labor in life cycle. Well-developed nervous system. Representatives of the arthropod group include: CHELICERATES

Spiders, scorpions, ticks, mites and the marine form called the horseshoe crab (Limulus).

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MILLIPEDES

These organisms are heterotrophs that inject enzymes into their prey to digest the prey’s body structures into a liquid. They then absorb the liquid. Some, such as the ticks, go straight for blood. Most are marine forms. This group includes the crayfish, shrimp, lobsters, crabs, barnacles and pill bugs. These organisms have 16-20 body segments, a carapace that covers the majority of the segments and appendages specialized for grabbing prey and holding in place while eating. Terrestrial, usually living under damp rocks or logs.

CENTIPEDES

These forms have a cylindrical body and are slow moving and nonaggressive. No fangs or venom glands. Terrestrial, usually living under damp rocks or logs.

CRUSTACEANS

INSECTS

These forms have a flattened body and are fast moving and very aggressive. They have fangs and venom glands. These arthropods are the ones that have three body segments for greater specialization: a separate head, thorax (chest) and abdomen. Insects are classified based (in part) upon: (1) number and type of wings and also the (2) type of mouthparts used for feeding.

Metamorphosis (complete or incomplete) is a common factor in insect lifestyles. Life cycles include social behavior, ability to communicate individually or within a group and division of labor within a social group.

PHYLUM ECHINODERMATA SPINY-SKINNED ANIMALS These organisms are bilaterally symmetrical when juveniles, but the mature forms specialize to become radially symmetrical. A coelom is present. Strictly marine forms. Some have capacity to regenerate (EX: starfish). Forms include starfish, brittle stars, sand dollars, sea urchins, sea lilies and sea cucumbers. Body specializations include and endoskeleton made of spines, muscles used for general body movements and tube feet used for locomotion. A water-vascular system powers the tube feet. Primarily sexual hermaphrodites.

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PHYLUM CHORDATA CHORDATES All members of this group are bilaterally symmetrical, have a true coelom and body segmentation. In addition to these specializations, they also all express these particular features at some point during their lives: NOTOCHORD TAIL GILL SLITS DORSAL NERVE CORD There are aquatic and terrestrial chordates. Their nutritional modes include herbivores, carnivores, scavengers and filter feeders. There are three major notochord groups. Two are invertebrate and the third includes all vertebrates. TUNICATES (SEA SQUIRTS) Marine, free swimming larva. Sessile adult.

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LANCELETS

Fish-like in appearance.

VERTEBRATES

Have endoskeleton of cartilage or bone and some sort of spinal (vertebral) support structure.

SURVEY OF VERTEBRATE ANIMALS

All vertebrates are chordates, so at some point during their life cycle they all have (even you!) a notochord, tail, nerve cord and gill slits. In addition to these chordate features, the vertebrate group of chordates has a vertebral column and endoskeleton made of cartilage or bone. Free-swimming vertebrate forms left fossils that date back to the Cambrian period of the Paleozoic era. Ostracoderms were among the first vertebrates. They were bottom dwelling jawless fish. The Placoderms developed next and improved the fishy body plan by adding jaws. CLASS AGNATHA JAWLESS FISH Includes free-living and parasitic forms. EX: The lampreys attach a sucker disc to the side of a fish and suck juices from the host’s body. CLASS CHONDRICHTHYES CARTILAGE FISH These organisms have a cartilaginous endoskeleton. The sharks are in this group along with skates, chimeras (ratfish) and rays. These organisms have placoid scales, which gave rise to teeth. CLASS OSTEICHTHYES These fish have an endoskeleton composed of bone.

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BONY FISH

Most successful of all vertebrates in terms of total numbers and diversity. Fish are cold-blooded (ectothermic) and have a two-chambered heat. Arose during the Silurian Period and diverged into three lines:

Lobe-finned fish

Most successful of the fish groups. Have gill cover called operculum, a swim bladder for buoyancy compensation and paired fins. Only one species still exists: the coelocanth.

Lungfish

Only three genera still exist.

Ray-finned fish

CLASS AMPHIBIA Three major groups: salamanders, frogs and toads.

DOUBLE-LIFE ANIMALS

They have a double life: part on land, part in water. The eggs of these organisms must be left in water so that when they hatch the larvae can swim and feed until their bodies specialize for limb development. These organisms have lungs and some also use their moist skin for respiration. CLASS REPTILIA REPTILES These organisms developed protected amniotic eggs and therefore did not have to return to water to lay their eggs. They were therefore, able to go anywhere on land they wanted to. They became extremely successful over time. The Mesozoic Era (which includes the Triassic, Jurassic and Cretaceous time periods) was known as the Age of Ruling Reptiles. Organisms in this group include snakes, lizards (Komodo dragons, etc.), turtles, alligators and crocodiles. Their heart has three chambers. These organisms are primarily cold-blooded (ectotherms). Reptilian skin is tough and resists water loss. These animals also have a copulatory organ that permits internal fertilization. CLASS AVES The key feature of this group is the modification of scales to form feathers.

BIRDS

Feathers (not flight) are the key specialization of the birds. Feathers provide surface protection, insulation and can be used by some for flight, These animals are warm-blooded (endothermic). They have a well-structured heart with four-chambers. This provides enough oxygen for metabolic efficiency. CLASS MAMMALIA MAMMALS The main feature of the mammals is that they feed their young with milk produced by mammary glands.

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All mammals have hair at some point during their life (whales have hair when they are only embryos). Mammals are warm-blooded (endothermic) and have a large well-developed four-chambered heart. The brain of a mammal is very specialized and contains lots of memory storing areas. There are three orders of mammals: 1-Monotremes 2-Marsupials 3-Placentals

Lay eggs and then feed their young with breast milk. Includes the platypus and spiny anteater. Young are born alive but then must migrate to their mother’s pouch in order to complete their embryonic development. Includes possums and kangaroos. Have placental membranes. These animals give birth to live young.

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