HO THE FRESHWATER MUSSEL. 



cavity it turns slightly downwards, runs over 

 the posterior adductor, and opens into the cloacal 

 cavity by a slit-like anus. 



The ventral wall of the rectum is folded so 

 as to form a prominent longitudinal ridge, the 

 typhlosole, projecting into its cavity. This 

 typhlosole is largest at the junction of the 

 rectum with the intestine. 



2. The liver is a large many-lobed digestive gland sur- 

 rounding the stomach and adjacent parts of the 

 oesophagus and intestine, and opening into the 

 stomach by several bile-ducts. 



G. The Reproductive System. 



The sexes are separate. The reproductive organs are 

 simple, and very similar in the two sexes. 



The ovary, or testis, is very large, and, when the sexual 

 products are ripe, forms the greater part of the visceral 

 mass between the foot and the kidneys. The ducts of each 

 side converge to the genital aperture which lies on the side 

 of the body immediately below the opening of the ureter. 



Tease a small portion of the ovary, or testis, in salt-solution 

 or glycerin ; cover, and examine it with low and high powers. 



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

 aperture, pass in large numbers into the space between 

 the^wo 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 

 rudimentary, as also is the foot. A long coiled filament 

 serves to anchor the glochidium to the gill of the parent. 



Take a female in which the outer gills are distended with 



