EXAMINATION OF UNIO OR ANODONTA. 285 



being large at the time of reproduction, and very small 

 immediately afterwards. They open into the cloacal 

 chamber, as already noticed. 



When the eggs pass out of the ovary they are conveyed 

 into the water tubes of the outer gills, which serve as 

 brood pouches, in which the developing eggs and young 

 are carried. 



XXVI. THE EXAMINATION OF TRANSVERSE 

 SECTIONS OF UNIO OR ANODONTA. 



THE general arrangement and relations of the parts in 

 Unio or Anodonta will be most easily understood by the 

 study of a series of transverse sections of a hardened 

 specimen. 



The sections which are figured are from Unio purpurea, 

 but any species will answer. 



An animal which has been preserved in strong alcohol 

 will 1>e found to be in fair condition for making sections, 

 but one which has been hardened in chromic acid is better. 

 The animal should be placed, alive, in its shell, in a quart 

 or more of one per cent chromic acid, and allowed to 

 remain for about forty-eight hours. After this time it 

 should be removed to seventy per cent alcohol, and al- 

 lowed to remain for a day or two. It may then be pre- 

 served in ninety per cent alcohol, and kept until it is 

 wanted. 



In order to cut the sections, the body must be carefully 

 removed from the shell without cutting or breaking it. 

 This may be done by forcing the valves of the shell far 

 enough apart to introduce the handle of a scalpel, which 

 may be used to force away the mantle and muscles from 

 their attachment to the shell. The body may now be 



