More Protostomes

<h1>Text Preview</h1> <p>Two of the most diverse and widespread groups of animals are the mollusks and arthropods. Because they’re everywhere in the environment, members of these protostome phyla are familiar to all of us from everyday encounters. They’re also known as sources of food and other benefits, and as pests of crops and carriers of disease. </p> <p>Mollusks have a great variety of body shapes. Underlying these forms is a similar basic body plan. The major internal organs of a mollusk are surrounded by a layer of specialized tissues called the mantle. Most mollusks are protected by a hard shell composed of calcium carbonate, which is secreted by the mantle. Some mollusks, however, have reduced or lost their shells in the course of evolution. Most mollusks also have a muscular foot, which is used for locomotion.</p> <p>Gastropods are the largest class of mollusks. This group contains the familiar snails and slugs. Land snails and slugs are the only terrestrial mollusks. Most gastropods have a distinct head with sensory organs. A large portion of the gastropod body undergoes a twisting or torsion during development, which leads to the spiral shells of snails.</p> <p>The bivalves include clams, oysters, scallops, and mussels. These mollusks have a two-part shell, hinged on one side. They’re mostly sedentary filter-feeders. They lack a distinct head, which is less useful for animals that don’t move about actively.</p> <p>Cephalopods are the mollusks best adapted for the rapid motion necessary to capture elusive prey. This class consists of squids, octopuses, and their relatives.</p> <p>Many fossil cephalopods possessed shells, but these have been reduced in size or lost by most modern species. A cephalopod’s mouth is surrounded by tentacles, used for capturing prey. Cephalopods have well-developed eyes and brains, and can move quickly by forcing a jet of water from the mantle cavity through a funnel-shaped siphon.</p> <p>The vast majority of animal species living today are arthropods. The bodies of these protostome animals are covered by a tough external skeleton, which is strengthened by the polysaccharide chitin. Arthropods have well-developed sensory organs and nervous systems. Arthropod bodies are segmented, with some segments bearing jointed appendages that are used for locomotion, feeding, sensing, and other functions. Arthropods are named for these appendages; the word “arthropod” means “jointed foot.” The organization and specialization of body segments and their appendages help distinguish the various groups of arthropods. </p> <p>Insects have by far the greatest species diversity of all the arthropods. Their numbers are mind-boggling, and it’s hard to overestimate their effects on the environment and on humanity. Insects are the only winged invertebrates. An insect’s body segments are fused into three regions: the head, the thorax, and the abdomen. The head bears sensory appendages including a pair of antennae.</p> <p>Many arthropods have a life cycle with immature stages that differ greatly in form from the adults. The caterpillar stage of butterflies and moths is an example.</p> <p>A short list of some important groups of insects illustrates the diversity of insect forms: beetles; butterflies and moths; ants and bees; flies and mosquitoes; grasshoppers and crickets; dragonflies; termites; and fleas.</p> <p>The chelicerates include spiders, scorpions, ticks, mites, and horseshoe crabs. These arthropods get their name from a pair of appendages near the mouth, called chelicerae. These are fangs or pincers used for capturing prey or for feeding. Most chelicerates have two body regions, with four or five pairs of walking legs attached to the foremost region. Millipedes and centipedes have long bodies with many segments, most of which bear pairs of legs. Millipedes feed mostly on plant matter, whereas centipedes hunt other small animals.</p> <p>The last group of arthropods we’ll look at are the crustaceans. This group includes many animals used by humans for food, such as crabs, shrimps, and lobsters. Barnacles and pill bugs are also crustaceans. Crustaceans are distinguished by a variety of appendages along their bodies, which may be highly specialized.</p> <p>This concludes our survey of the protostomes. Next we’ll move closer to our own branch of the tree of life as we examine the deuterostomes.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>