196 



THE SEA SHORE 



on your left. Knowing the exact uses of these few terms you are 

 in a better position to understand the descriptions of bivalves, and 

 to locate the exact situations of the various internal organs named 

 in such descriptions. 



A great deal of the internal anatomy of a bivalve mollusc may 

 be made out by easy dissections, and although the structure of the 



different species varies in several 

 details, the general characteristics 

 of the group are practically the 

 same in all and may be gathered 

 by the examination of a few speci- 

 mens. 



For this purpose the shell 

 should be prised open by means 

 of some flattened but blunt im- 

 plement, such as the handle of a 

 scalpel, and then, after inserting 

 a piece of cork to keep the valves 

 apart, gently remove the mantle 

 lobe from the valve which is held 

 uppermost with the same imple- 

 ment, being careful to separate 

 it from the shell without doing 

 any damage to the soft structures. 



Separating the mantle from the shell in this way we meet with one 

 or more hard masses of muscle that are joined very firmly to the 

 latter. These are the adductor muscles that pass directly from 

 valve to valve, and on cutting them through close to the uppermost 

 valve, the latter can be raised so as to expose the body of the 

 animal, mostly hidden by the overlying mantle lobe. 



Before raising the upper mantle lobe we observe the heart, on 

 the dorsal margin of the body, near the hinge of the shell, situated 

 in a transparent cavity (the pericardium) containing a colourless 

 fluid. It consists of at least two cavities a thick- walled ventricle 

 and a thin-walled auricle, and its slow pulsations may be watched 

 with or without the use of a hand lens. On opening the pericar- 

 dium the heart is still better seen, and if we carefully cut into the 

 thick-walled ventricle we find a tube running completely through 

 its cavity. This is the rectum the last part of the digestive tube, 

 that commences at the mouth, and terminates in a cavity at the 

 posterior end communicating with the exhalent siphon. 



FIG. 131. A BIVALVE SHELL 

 (Tapes virgineana) 



a, anterior : p, posterior ; I, left valve ; 

 r, right valve ; u, umbo, on dorsal side 



