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Phylum Mollusca Molluscs are bilaterally symmetrical eumetazoans. They have a true coelom (eucoelom) which is formed by the splitting of embryonic mesodermal masses (schizocoely). Development is protostomous. An important characteristic of most molluscs is the head-foot region. Most molluscs are strongly cephalized; that is, they have a well-developed head, in which is located a mouth and a concentration of nervous and sensory functions. Adjacent to the head is a large, muscular foot formed from the ventral body wall. Used primarily in locomotion, the surface of the foot is sometimes ciliated and laden with numerous mucous glands. Another characteristic of most molluscs is the mantle. This sheath of tissue is formed from the dorsal body wall. It surrounds the mantle cavity, which houses the gills or lungs if they are present, and its surface may assist in gas exchange. The mantle also secretes the shell in those forms that possess one. Internally, molluscs have a complicated digestive system, with a mouth in the head and the anus emptying into the mantle cavity. A rasping organ used in feeding, the radula, is present in all groups except bivalves and Aplacophora. The radula is usually toothed and its structure may be very complex. It is adapted to a wide variety of feeding styles in different species, including scraping, tearing, stabbing, and cutting. Molluscs have an open circulatory system (except cephalopods, in which it is closed), complete with a heart, blood vessels, and respiratory pigments. Gas exchange is via gills, lungs, or the body surface. Excretion takes place by means of kidneys that, like the digestive tract, pump waste into the mantle cavity. A fairly complicated nervous system is present, including several ganglia and a system of nerves; sensory and nervous systems are cephalized in at least some kinds of molluscs. Some molluscs have complex, extremely sensitive eyes. The majority of molluscs have a shell of some kind. This calcareous structure is secreted by the mantle, and in living snails it is covered by a horny layer called a periostracum. Its shape, size, and color are widely used by taxonomists, and they has also made molluscs a popular pursuit of collectors. Further, shells decay slowly and fossilize well, and the fossil record of molluscs is excellent and ancient. Most molluscs are dioecious. Many pass through free-swimming larval stages, called trochophore and veliger larvae. Molluscs are an extremely diverse group of organisms. Over 50,000 living species have been named, making Mollusca second only to the Phylum Arthropoda in number of species known. The majority of molluscs are marine, but large numbers of species occupy freshwater and terrestrial habitats. Molluscs are also extremely diverse in their food habits, ranging from species that graze on microscopic algae, to those that eat the leaves of terrestrial plants, to predators that capture fish and other molluscs. Many species of molluscs are important to humans. A large number of bivalves and some snails are important sources of protein. Oysters produce pearls. Other species are pests in gardens and to crops. Some are essential components in the life cycles of human parasites, including devastating diseases such as schistosomiasis. And a few, such as oyster drills, cause problems by preying on other molluscs that are important to us. Finally, molluscs play a wide variety of essential ecological roles. As common herbivores, they can have a significant impact on the plant species present in an area. As predators, they may have a similar effect on animals, especially other molluscs. As prey, they provide food for a large number of organisms, including many vertebrates (some of which, such as sea otters, are strikingly specialized for feeding on them). An example of the ecological (not to mention economic) impact of molluscs is given by the recent introduction of zebra mussels to the Great Lakes. Zebra mussels live in large numbers on hard surfaces, and they feed by filtering particles from the water. The effectiveness of their feeding is remarkable; water clarity in some of the Great Lakes has improved dramatically in the years since zebra mussels first appeared. Unfortunately, the same material that the mussels are efficiently removing is the basis of a complicated food chain for other species, including economically-important game fishes. Fisheries biologists in the Great Lakes states are very concerned about the future of the species they tend. The impact of these changes on less conspicuous species, such as the many kinds of native bivalves, is not known, but their populations are certainly at risk. http://www.kent.wednet.edu/staff/rlynch/sci_class/chap14/mollusc.html http://orion1.paisley.ac.uk/courses/Tatner/biomedia/units/moll1.htm Annelida Phylum Annelida The Segmented Worms "Although a spacious, fluid-filled coelom provided an efficient hydrostatic skeleton for burrowing, precise control of body movements was not possible in the earliest coelomates. The force of muscle contraction was carried through the body by the fluid in the undivided coelom. This defect was remedied when a series of partitions (septa) evolved in the ancestral annelids. When the septa divided the coelom into a series of compartments, components of most other body systems, such as circulatory, nervous, and excretory, were repeated in each segment. This body plan is known as metamerism." Hickman, C.P. and L.S. Roberts. 1994. Biology of Animals, Sixth Edition. Wm.C.Brown Publishers: Dubuque, Iowa. Page 526. Major Characteristics of Phylum Annelida 1. Body metamerically segmented; symmetry bilateral; 2. Body wall with outer circular and inner longitudinal muscle layers; outer transparent moist cuticle secreted by epithelium; 3. Chitinous setae, often present on fleshy appendages called parapodia; setae absent in leeches; 4. Coelom (schizocoel) well developed and divided by septa, except in leeches; coelomic fluid supplies turgidity and functions as hydrostatic skeleton; 5. Blood system closed and segmentally arranged; respiratory pigments (hemoglobin, hemerythrin, or chlorocruorin) often present; 6. Digestive system complete and not metamerically arranged; 7. Respiratory gas exchange through skin, gills, or parapodia; 8. Excretory system typically a pair of nephridia for each metamere; 9. Nervous system with a double ventral nerve cord and a pair of ganglia with lateral nerves in each metamere; brain, a pair of dorsal cerebral ganglia with connectives to cord; 10. Sensory system of tactile organs, taste buds, photoreceptor cells, and eyes with lenses (in some); 11. Hermaphroditic or separate sexes; asexual reproduction by budding in some; Classes of Annelids: Class Polychaeta - The polychaetes are the largest class of annelids, with more than 10,000 described species, most of them marine. They differ from other annelids in having a well-differentiated head with specialized sense organs; paired, paddlelike appendages (parapodia) on most segments, and no clitellum. Example: Nereis (the clamworm) Class Oligochaeta - The more than 3,000 species of oligochaetes are found in a great variety of sizes and habitats. Most are terrestrial or freshwater forms, but some are parasitic and a few live in brackish or marine water. With few exceptions, they bear setae, which may be long or short, straight or curved, blunt or needlelike, or arranged singly or in bundles. Examples: Lumbricus (earthworm) {go to Earthworm Dissection Home Page} for information about earthworms on another o websites! Class Hirudinea - Leeches, numbering over 500 species, are found predominantly in freshwater habitats, but a few are marine, and some have even adapted to terrestrial life in warm, moist areas. They are usually flattened dorsoventrally, and are found in a variety of colors - black, brown, red, or olive green. Leeches have a fixed number of segments, usually 34, and typically have both an anterior and a posterior sucker. http://orion1.paisley.ac.uk/courses/Tatner/biomedia/units/anne1.htm