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INTRODUCTION TO KEY— GENERAL ANATOMY 

 OF BIVALVES 



In order to use the key the following facts concerning the structure 

 of these animals mnst be borne in mind. All are protected by two 

 similar shells or valves (hence the term bivalves), which are formed on 

 the right and left sides of the animal, the back or dorsal side being 

 that where the valves are joined together. This shell which is so 

 striking a feature of tliese animals is the product of the mantle, a soft 

 structure also characteristic of this group. This covers the animal as 

 the flyleaves cover the body of a book and by its activity secretes the 

 valves of the shell which thus come to occupy the position of the 

 covers of the book. If we think of a book with limp leather covers 

 wliich bend over to meet each other around the edges and imagine the 

 flyleaves doing the same thing, the picture is a very complete one. 

 In the simplest form of the mantle the edges are free except on the 

 back where the hinge is located, corresponding to the arrangement in 

 a book, and the sea water may then enter the cavity enclosed by the 

 mantle at almost any place; this is the condition, for instance, in the 

 oyster. In all bivalves the food consists of microscopic plants and 

 animals obtained from the water which is made to flow through the 

 mantle cavity by the beating of myriads of fine hair-like cilia that 

 whip the water along. From the water they also obtain by means of 

 the gills tlie oxygen necessary for respiration. 



In most of the bivalves, however, the mantle edges are not every- 

 where open but are fused together at certain points. Two openings 

 are thus formed at the posterior or hinder end of the body and through 

 these the water currents enter and leave the body, the inhalent opening 

 lying nearer the lower side and the exhalent nearer the upper or 

 hinge side. The mantle surrounding these openings is often, as in 

 the case of the common clams, developed into two tubes, sometimes 

 separate and sometimes united so as to appear as one tube but with 

 two cavities, and it is through these that the clam obtains its water 

 when buried in the sand or mud. These form the "neck" or, more 

 properly, siphons of the clam ; it will be seen that these are in no way 

 related to the neck of other animals, the clam's "head" being at the 

 opposite end. Of the other organs of the body inclosed l)y the mantle, 

 it will be sufficient to mention the foot, a muscular organ which can 

 be protruded between the edges of the mantle and shell either at the 

 anterior or "head" end, or at the ventral side, and by means of which 

 the animal burrows or moves about. 



The inner surface of the empty shell shows certain marks where the 

 soft portions of the body were attached, so that from the shell alone 

 many facts concerning the anatomy can be made out. There are 

 usually two large scars near either end of the valve marking the points 

 where the two muscles that close the shell were attached, the posterior 

 adductor mvscle near the siphonate end and the anterior adductor 

 muscle near the opposite or anterior end (see figure 3, page 8). 

 Though in most forms these two muscles and their scars are of about 

 the same size, there are species, such as the common mussel, in which 

 one is much reduced in size (see figure 5, page 10), and others, such 



