The Animal Kingdom: Invertebrates

The animal kingdom consists of multicellular organisms that feed by ingestion. On the simplest level, they are made up of tissues. As the animals become more complex, these tissues form organs. At the highest level, these tissues and organs form organ systems. The animal kingdom can be divided into vertebrates, or those with backbones, and invertebrates, those without backbones. Every living thing is given a scientific name made from two Greek or Latin words. The first is the genus name, and the second is the species.

Invertebrates include Sponges, Flatworms, Coelenterates, Echinoderms, Roundworms, and Segmented Worms, Mollusks, and Arthropods (Crustaceans, Arachnids, and Insects).

Sponges are in Phylum:Porifera. These animals attach permanently to underwater objects. They have two cell layers with a jellylike substance in between, a pore-covered body with a skeletal structure for support, no head or mouth, no tissues or organs, and digest using collar cells.

Coelenterates (Phylum:Cnidaria) may have one or two main body forms; cylinder-shaped polyp or umbrella-shaped medusa. This phylum includes coral, jellyfish, sea anemones, and sea fans. These have two cell layers separated by a jellylike substance, radial symmetry, tentacles with stinging cells, and a digestive cavity with one opening.

Flatworms (Phylum:Platyhelminthes) are parasitic. Tapeworms and planarians are in this phylum. Flatworms have bilateral symmetry, three cell layers, a digestive cavity with one opening and no circulatory or respiratory systems.

Roundworms (Phylum: Nematoda) are mainly parasitic and live in soil, water, or dead tissue. Nematoda includes trichinae, hookworms, and pinworms. Roundworms have a tubular body with bilateral symmetry, a digestive tract with two openings, and reproduce sexually.

"Wheel Animals" (Phylum: Rotifera) live in lakes, rivers, oceans, and streams. They have a cylindrical or vase-shaped body with a crown of cilia on the head, and a complete digestive tract. A brachionus calyciflorus is an example of a wheel animal.

Segmented worms (Phylum: Annelid) include earthworms, sandworms, and leeches. Segmented worms have three cell layers, coelem with a tube-within-a-tube body plan, a pair of bristles, or seta, on each body segment, and move by contraction of circular and longitudinal muscles.

Mollusks (Phylum: Mollusca ) are mainly aquatic and ge their food and oxygen from the water that flows through their bodies. There are three major classes of mollusks. These are Pelecypoda, Gastropoa, and Cephalopda. Mollusks have three distinct body parts: head-foot, visceral mass, and mantle, and a soft body protected by on two shells secreted by a mantle. Pelecypoda have a shell consisting of two hinged valves and a muscular foot. This class includes clams, oysters, and mussels. Gastropoda are "belly-footed" with a one-coiled shell. Snails and slugs are in this class. Cephalopoda have a well-developed head and foot divided into many armlike tentacles. Octopi and squid are included in this class.

Arthropods (Phylum: Arthropoda) have jointed appendages, segmented bodies, and exoskeletons. There are five major classes. Arachnida have eight legs. This class includes spiders and scorpions. Crustacea includes crabs, lobsters, and shrimp. These have chewing jaws and a pair of appendages on each segment of the thorax. Chilopoda are centipedes. Centipedes have one pair of legs per body segment. Diploda are millipedes which have rounded bodies and two pairs of legs per body segment. Insecta includes bees, beetles, butterflies, flies, and grasshoppers. Insects have three distinct body parts-head, thorax, and abdomen-an three pairs of legs.

Echinoderms (Phylum: Echinodermata) include starfish, sand dollars, brittle stars, sea urchins, and sea cucumbers. Echinoderms have a water-vascular system, an internal skeleton, radial symmetry, spiny skin, and tube feet.