Phylum Arthropoda

    Arthropoda is the most diverse and numerous animal phylum. Insects, spiders, and crustaceans—which include lobsters, shrimp, and crabs—constitute the major arthropod groups. The name Arthropoda means “jointed feet”; arthropods have jointed appendages and, like annelids, exhibit segmentation. Insects and crustaceans have three body segments consisting of the head, thorax, and abdomen, while arachnids only have two body segments. Arthropods are unique among animals in having a hard exoskeleton made of chitin. The arthropod nervous system resembles the annelid nervous system, with a simple brain, a ventral nerve cord, and smaller ganglia within the various body segments. However, many arthropods have very highly developed sensory perception, including hearing organs, antennae, and compound eyes. Arthropods have an open circulatory system, a full digestive tract, and structures called Malphigian tubules to eliminate waste.

 

    Phylum Echinodermata

    The name Echinodermata means “spiny skin,” and this phylum includes spiny marine animals such as starfish, sea urchins, and sand dollars, all of which exhibit radial symmetry. Echinoderms have several features, including an endoskeleton that secretes a spiny skin and an unusual vascular system of water-filled vessels that regulates the movement of their many tube feet and also permits the exchange of carbon dioxide for oxygen. Echinoderms have a very simple nervous system, with a ring of nerves around their mouth and no brain. Some echinoderms filter food out of the water, while others, like starfish, are carnivorous predators or scavengers. Despite their primitive appearance, patterns in early embryonic development strongly suggest that echinoderms are most closely related to the chordates, the animal phylum that developed most recently in evolutionary time.

 

    Phylum Chordata

    Human beings belong to Chordata, the phylum that evolved most recently in the animal kingdom. Chordates have three embryonic tissues, a complete digestive tract, and well-developed circulatory, respiratory, and nervous systems. Several features distinguish chordates from all other animal phyla. The primary feature, for which chordates are named, is the notochord, a tubular rod of tissue that runs longitudinally down the back. Just above the notochord runs a single, hollow nerve cord, the center of the nervous system. Other animals, such as earthworms, also have nerve cords; however, these run in ventral pairs along the belly and are not hollow. Two other features, gill slits and tails, are present in all chordates during embryonic development but disappear by adulthood in many members of the phylum.

There are two groups of chordates, subphylum Urochordata and subphylum Vertebrata. The former subphylum includes invertebrate marine animals such as tunicates and lancelets, and almost never appears on the SAT II Biology. Much more important for the test are the vertebrates.