96 THE FRESHWATER MUSSEL 



G. The Reproductive System. 



The sexes are separate. The reproductive organs are 

 simple and very similar in the two sexes. 



The generative gland, whether ovary or testis, is very 

 large, and, when the sexual products are ripe, fills up the 

 greater part of the visceral mass between the foot and the 

 kidney. The ducts converge on each side to the genital 

 aperture, which lies on the side of the body immediately below 

 the opening of the ureter. 



In the female the eggs, after they escape from the genital 

 aperture, pass in large numbers into the space between 

 the two lamellae of the outer gill, which they distend very 

 greatly. Here they develop into embryos known as glochidia. 

 which are in many respects very unlike the parent. They 

 have bivalved shells, each valve of which is triangular and 

 incurved at its apex so as to form a sharp tooth, the teeth of 

 the two valves constituting an efficient pair of pincers. There 

 is only a single adductor muscle ; the gills are absent or rudi- 

 mentary ; the foot also is rudimentary, and from it a long 

 coiled filament, the byssus, arises, which serves to anchor the 

 glochidium to the gill. 



Tease a small portion of the generative gland in salt 

 solution or glycerine ; cover, and examine it with low and high 

 powers. 



Take a female in which the outer gills are distended with 

 embryos : cut the gill across ; remove some of the embryos, 

 mount them on a slide in water, and examine them with a 

 microscope. 



III. EXAMINATION OF TRANSVERSE SECTIONS. 



Several points in the anatomy of the mussel, and notably 

 the relations of the gills, kidneys, and heart, are best studied 

 by means of a series of transverse sections. 



F.or this purpose take a good-sized specimen, and put it 

 into \ per cent, chromic acid, wedging the valves slightly open 

 so that the acid may have free access to the branchial cavity. 



