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Unformatted text preview: PALEO LABORATORY # 6, PAGE 1 PALEO LAB # VI - MOLLUSCS A. Phylum Mollusca The molluscs constitute one of the main groups of invertebrates. Here belong the endless variety of marine, freshwater, and terrestrial snails; oysters, mussels, scallops, and many other sorts of clams living in the sea and in streams, lakes and ponds; squids, cuttlefishes, octopuses, the pearly nautilus, and a host of extinct related forms. Among the 60,000 or so living species of molluscs are forms ranging in adult size from snails less than 1/16 inch long to the giant squids of the Atlantic Ocean which attain a length of 50 feet. Some molluscs are active swimmers, some float or drift passively about as planktonic organisms, some burrow into mud or sand, or attach themselves solidly to almost any firm foundation. However, most crawl about by means of a muscular organ of locomotion, termed the Foot. The chief distinguishing features of molluscs typically are: normally elongate, unsegmented (versus arthropods, etc.), bilaterally-symmetrical organization of the body; enclosure of the viscera by a body wall, of which the lower part (Foot) is modified for locomotion and the upper part (Mantle) hangs down as a fold enclosing the free space between it and the body; concentration of sensory structures in a head (except bivalves); and special characters of the digestive and nervous system. The mantle contains cells which secrete the shell. In all molluscs the shell consists primarily of calcium carbonate. Most molluscs secrete shells of aragonite, although some form their shells of calcite and still others have shells composed of both minerals. Normally the shell is external, but has become completely internal on some forms. Any attempt at a morphological or anatomical definition of the Phylum Mollusca results in so many qualifications and exceptions as to be meaningless, as there are no synapomorphic characters that link all of the molluscs. Most paleontologists believe the molluscs may be monophyletic on the basis of paleontology, stratigraphy, zoology, embryology and biochemistry. It is believed that the molluscs are descended from a single lineage of bilaterally symmetrical, unsegmented, acoelomate animals that developed a dorsal exoskeleton in the form of an organic (proteinaceous) cuticle, an array of calcareous spicules or a single continuous calcareous shell. In addition to these derived features, molluscs inherited an open-ended gut, dorso-ventral muscles and a developmental program that begins with the spiral cleavage of a fertilized egg. Despite the evidence of a single ancestor for the molluscs, several recent workers have proposed dividing them into more than one distinct phylum. Stay tuned for further developments....
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This note was uploaded on 01/16/2012 for the course GEOL 305 taught by Professor Dr.phillipmurphy during the Fall '10 term at Tarleton.
- Fall '10