56 



LAND AND FRESH-WATER SHELLS. 



the former portion is more reddish than the latter. The genital 

 aperture opens just below the orifice of the ureter. The ova in 

 the female do not pass direQtly out of the body, but congregate 

 in large numbers between the two lamellae of the outer gill, where 

 they develop into peculiar larval forms — which were considered 



Fig. 23. — A, Glochidium-larva still within the egg; B, Glochidiuni viewed from the side 

 showing the teeth on its valves. 



parasites of the mussel by Rathke and Jacobson — known as 

 Glochidia. Each valve of a glochidium-shell is shaped like an 



Fig. 24.— Shell of Glochidium widely opened, showing the single adductor muscle. 



equilateral triangle, with the apex incurved, and produced into a 

 sharp saw-edged tooth. A single adductor muscle is present ; 



