GENERAL ZOOLOGY 



poda, a small group of unfamiliar marine animals called "tooth shells"; 

 and the class Cephalopoda, the squids, cuttlefishes, and nautili. 



The majority of mollusks are free-living animals, adapted to a creeping or 

 burrowing existence and provided with a protective shell into which the ex- 

 tensible soft parts can be withdrawn. Such mollusks as the cephalopods, 

 however, are modified for a more active, free-swimming habit. Most mollusks 

 are marine animals and are abundant in shallow waters but not at great 

 depths. The sea seems to have been the primitive habitat, but many species 

 of gastropods and pelecypods are found in fresh water, and gastropods are 

 common also on land. The more representative types were classified as 

 Mollusca by Aristotle, Linnaeus, and Cuvier, but in the early classifications 

 this group included many animals which have since been distributed among 

 other phyla. Many diflferent kinds of mollusks are used as food today, and 

 archaeological evidence indicates that shellfish were a very important item in 

 the diet of many primitive humans. 



In this chapter we shall examine particularly the characteristics of bivalve 

 mollusks such as the clam and the fresh-water mussel, as animals typical of 

 this grade of organization, to be compared with the hydra, the planarian, 

 the earthworm, and the vertebrate. 



THE CLASS PELECYPODA 



The Clam or Fresh -Water Mussel: General Structure and Actimties. 



The following account is applicable to any of the common marine clams, 

 such as Venus mercenana, or to the fresh-water mussels such as species of the 

 genera Lampsilis or Anodonta. The shell is composed of two valves, fitted 

 together at the dorsal side to form a hinge which is covered externally by a 

 tough, elastic hinge ligament. The dome-like part of each valve, lying near 

 the hinge, is termed the umbo (plural, umbones); this is the oldest part of 

 the shell, as indicated by the rings of growth which surround the umbo and 

 mark the successive outlines of the growing shell. The valves are lateral; the 

 hinge ligament is dorsal; the gape of the valves is ventral. The umbones 

 generally lie just anterior to the midline of the shell, although between 

 different species of bivalves this characteristic is subject to much variation. 

 Looking at a clam with its valves intact reveals nothing of the living animal 

 itself, except as the foot may be thrust out between the margins of the shell, 

 or the tube-like siphons extended posteriorly. 



Removing the shell reveals the external surface of the clam. Closely ap- 

 pressed to the internal surface of each valve is a flap of the mantle (Fig. 13.1). 

 On each side, the mantle represents a sheet-like outgrowth of the dorsolateral 

 body wall. Its functions include the secretion of the shell, the various layers 

 of which are produced by specialized tracts of glandular cells at the edges of 

 the mantle and on its external surface. Passing through the mantle on both 

 sides are the fibrous masses of the anterior and posterior adductor muscles; these 

 course directly from one valve to the other and are attached to the valves at 



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