Evolution of the Animal Body Plan Part 1 - Invertebrate Animals E. Q. – How has the animal body plan changed over time?

Phylum Porifera (Sponges) Sponges are the simplest animals. Their bodies consist of a few types of cells grouped together. They do not have symmetry, germ layers, or any other characteristics of more highly evolved animals. Sponges are filter feeders that live off of bacteria and other microscopic organisms. Water flows through the body of the sponge past choanocytes, also called collar cells. These cells trap the food particles and ingest them by phagocytosis. Once food has been ingested by the choanocytes, it passes to wandering amoebocytes by diffusion. The amoebocytes move through the rest of the sponge, where the food passes to remaining cells again by diffusion. Sponges maintain their shapes because they have stiff fibers (spicules) or flexible fibers (spongin). Sponges reproduce asexually or sexually. Sponges are mostly marine, but there are a few freshwater species. - Development of Multicellularity

Phylum Porifera

Phylum Cnidaria (Hydras, Sea Anemones, Jellyfish, Corals) Most cnidarians are marine, but there are also some freshwater species, such as hydras. Most are sessile or slow- moving, but are excellent predators. Unlike Poriferans, cnidarians have symmetry (radial), two germ layers (endoderm and ectoderm), and a single digestive opening surrounded by tentacles carrying stinging cells (cnidocytes). Cnidarians have two basic body plans—polyp or medusa. Because they are radially symmetrical, cnidarians do not have a head. The Cnidarian’s central body cavity is its gastrovascular cavity, which allows the animal to eat prey larger than microscopic organisms. Cnidocytes, another phylum characteristic, contain a venomous barb that paralyzes prey. The tentacles then pull the prey into the mouth. - Development of tissue layers and symmetry

Phylum Cnidaria

Phylum Platyhelminthes (Planaria, Flukes, Tapeworms) There are about 20,000 species of Phylum Platyhelminthes. They inhabit freshwater and marine environments and include free-living as well as parasitic forms. One of the most familiar platyhelminths is the free-living planarian Dugesia, which feeds along rock surfaces and the undersides of vegetation. The animal is acoelomate like cnidarians. It does exhibit major evolutionary advances, however. Platyhelminths are bilaterally symmetrical, and have evidence of cephalization. They have three germ layers and distinct organs and organ systems. They reproduce asexually and sexually. Most platyhelminths are hermaphrodites. Tapeworms are adapted to a parasitic life. They lack digestive cavities or a means of locomotion, but do have an organ of attachment, the scolex. They also produce abundant numbers of offspring, since each of their many posterior segments (proglottids) contains ovaries and testes. - Development of bilateral symmetry, cephalization, and organs

Phylum Platyhelminthes

Phylum Nematoda The phylum Nematoda is made of roundworms, which are commonly known as nematodes. Some are parasitic, such as roundworms and hookworms. One, Trichonella sporalis, which can be transmitted to humans by undercooked meat, causes the disease trichinosis. Many others are free living, such as nematodes found in soil and vinegar eels. All are bilaterally symmetrical, and have similar body plans, that show evolutionary advances over platyhelminths. Nematodes have some degree of cephalization, with two nerve cords. They also have two openings to the digestive tract, which allows the food to move in one direction through the gut. Nematodes have a body cavity that forms between the endoderm and the mesoderm called a pseudocoelom. This allows for independent movement of the gut and the body wall. Nematodes reproduce sexually, and the sexes are separate. Fertilization is internal, and thousands of offspring are produced. - First sign of a body cavity ( pseudocoelom)

Phylum Nematoda

Phylum Mollusca This phylum includes clams, oysters( bivalves), snails and slugs (gastropods), and octopi and squids (cephalopods). These animals are coelomate but unsegmented, with soft bodies. They have three general body regions: the mantle, the foot, and the visceral mass. All mollusks have a strong muscular foot (different in different mollusks), used for creeping, swimming, or catching prey. Many mollusks have an external shell, which is secreted by their mantle. Most mollusks are aquatic, but some are terrestrial. In aquatic mollusks, gills are used for respiration. Most mollusks have an open circulatory system with a heart. Bivalves are filter feeders, with powerful adductor muscles that hold their two shells together. Cephalopods have feet modified into tentacles and well-developed nervous systems. Mollusks other than bivalves have a serrated radula used to break up large food particles. - Development of a true coelom

Phylum Mollusca

Phylum Annelida Members of the phylum Annelida include many marine worms, earthworms, and leeches. Their major evolutionary advance is that their bodies are segmented. They have a head with a mouth, and a terminal segment with an anus. They also have a true coelom, which develops completely within the mesoderm. Their digestive tract runs the length of the worm and contains specialized organs such as the pharynx, esophagus, crop, gizzard, and intestine. Annelids have segmental bristles (setae) and both longitudinal and circular muscles lining the coelom, which are used for swimming, crawling, or burrowing. Annelids have complex, closed circulatory systems, with multiple muscular pumps (hearts). Their nervous systems have a simple brain and a ventral nerve cord that connects it to other areas of the body. The annelids have nephridia in each segment, which excrete nitrogenous wastes. - Development of body segmentation

Phylum Annelida

Phylum Arthropoda (Crustacea) Arthropods are the most successful animal phylum. Most animals on Earth are arthropods. There are over 1,000,000 different species. An arthropod has a segmented body with a chitinous exoskeleton and jointed appendages. In order to grow, arthropods must shed their exoskeleton periodically. Their segments do not repeat, like those of annelids, but instead are highly specialized for feeding, locomotion, reproduction, and sensing the environment. An arthropod has an open circulatory system with a well- developed digestive system. Phylum Arthropoda includes millipedes and centipedes, crustaceans, arachnids, and insects. Most crustaceans are aquatic, but a few, like the rolly-polly are terrestrial. Other crustaceans are crabs, lobsters, crayfish and shrimp. Crustaceans generally have five pairs of walking legs and two pairs of antennae. Crustaceans have mineralized exoskeletons, which make them sturdier than the exoskeletons of other arthropods. - Development of jointed appendages, specialized segments

Phylum Arthropoda (Spiders, Scorpions, Millipedes, Centipedes) Millipedes and centipedes have cylindrical bodies made of many repeating segments with distinct heads and posteriors and a chitinous exoskeleton. Millipedes have two pair of jointed walking legs per body segment, are non-venomous, and eat decaying plant matter. Centipedes are similar in appearance, but have only one pair of walking legs per segment. They are predators and scavengers. They are venomous. Their first pair of legs is used to inject poison into their prey. Arthropods that have eight legs include spiders, scorpions, ticks, and mites. They are also called arachnids. In arachnids, the head and the thorax are usually fused together. Many species are arachnids are venomous. Ticks are blood- sucking parasites that are vectors for many diseases. Scorpions are one of the earliest terrestrial animals. Spiders breathe using a book lung or tracheae, and can spin silk.

Phylum Arthropoda (Insecta) Many of the adaptations of insects that have made them so successful are also characteristic of other arthropods. All insects have a segmented body with an exoskeleton of chitin and jointed appendages specialized for various uses. The body of an insect is divided into three regions: the head, the thorax, and the abdomen. All insects have three pairs of walking legs, which are attached to the thorax. If wings are present, they are also on the thorax. The abdomen contains 9-11 segments, and in adults, it has no legs nor wings attached. Many insects, like other arthropods, have compound as well as simple eyes. An insect has an open circulatory system with a muscular heart to circulate nutrients. Gas exchange is done by the tracheae and spiracles. Insects and spiders use Malpighian tubules to excrete metabolic wastes. Insects have a brain and ventral nerve cord.

Phylum Arthropoda

Phylum Echinodermata Echinoderms have spiny skins that are made of hard, calcified plates that form a kind of endoskeleton just below the epidermis. All echinoderms live in saltwater or brackish water. They are radially symmetrical as adults but bilaterally symmetrical as larvae. Echinoderms are not segmented, and they lack a head. Echinoderms have a modified coelom called the water vascular system that helps them move. When an echinoderm feeds, it extends its gut outside of its body. Sexes are separate in echinoderms, and fertilization is external. Echinoderms have no centralized nervous system. Examples of echinoderms include starfish, sea urchins, sand dollars, brittle stars, and sea cucumbers. Echinoderms are deuterostomes, unlike the other invertebrate phyla. Phylum Chordata is the most advanced animal phylum. - Development of deuterostomy in place of protostomy

Phylum Echinodermata

E. Q. – How has the animal body plan changed over time? With respect to each of the following concepts, tell how the animal body plan has evolved:  Tissue layers  Type of symmetry  Cephalization  Development of organ systems  Body cavity  Body segmentation  Appendages  Embryonic development  Nervous system development