54 COMPARATIVE ANATOMY 



living specimen is kept in an aquarium with sufficient sand or 

 mud at the bottom, it will be seen to bury the anterior and 

 blunter part of its body, leaving the posterior more pointed 

 part sticking up in the water. If a little indigo or other 

 colouring matter is dropped into the water near the posterior 

 end it will be seen to be swept into the cavity of the shell by a 

 current setting inwards just below the pointed end of the shell, 

 and after a while it will emerge from an aperture just above 

 this and be carried away by a current setting outwards. There 

 is, then, a constant stream of water passing through the cavity 

 contained in the shell, the points of entrance and exit of the 

 current being situated close together at the hind end of the 

 body. The current subserves nutrition as well as respiration, 

 for the fresh-water mussel feeds entirely on minute animal and 

 vegetable organisms contained in the water. 



An empty shell should be examined before proceeding to the 

 study of the internal structure. The shell is bivalve, consisting 

 of two pieces called valves, which are equal in size, similar, 

 and occupy the right and left sides of the animal. Each shell is 

 irregularly ovoid in outline, the anterior end being more 

 rounded, the posterior end more tapering. The line along 

 which the two valves are hinged together is nearly straight, and 

 corresponds to the dorsal surface of the animal. The exterior 

 of each valve is of a dark olive green colour, and marked by a 

 number of concentric lines parallel with the free margin. 

 These represent successive stages of growth, and start from a 

 more or less prominent projection situated close to the hinge 

 line, and much nearer the anterior than the posterior end of 

 the shell. The prominence is called the umbo, and is the 

 oldest part of the shell. The internal surface of the shell is 

 white and pearly. If a piece of the shell is ground down on a 

 stone it can easily be seen to be composed of three layers viz. 

 an outer organic layer called the periostracum, formed of a 

 substance called conchiolin, which projects as a dark and 

 flexible rim all round the margin of the shell ; a middle or pris- 

 matic layer, formed of the same substance, conchiolin, impreg- 

 nated with salts of lime arranged in prisms, and an inner layer 

 composed of mother-of-pearl or nacre, hence called the nacreous 

 layer. The inner surface of each valve exhibits certain well 

 defined depressions, which mark the attachment of the adductor 

 and retractor muscles to be described presently. The anterior 



