172 



accessory organs open : the spermotheca or receptaculum 

 seminis (a globular sac with a long tube), a tuft of tubular 

 mucous glands, and a dart sac. The male duct or vas defer- 

 ens is long and thin, and it leads into the muscular pro- 

 trusible penis, which has at its base a long tubular flagellum 

 and a retractor muscle. The vagina and the penis open 

 together at the genital pore. 



In the ovotestis sperms are produced first and ova 

 later : this form of hermaphroditism is known as protandry. 

 The sperms pass into the flagellum, and are there massed 

 together (spermatophores). Limy darts, formed in the dart- 

 sac, are forcibly shot out, through the genital aperture, 

 by the pairing snails ; copulation then takes place, and 

 the spermatophores received by each snail are passed into 

 the spermotheca. The ova, produced later, are fertilised 

 at the Upper end of the female portion of the common 

 duct, by the sperms received from another snail (cross- 

 fertilisation). The eggs are coated with limy mucus se- 

 creted by the mucous glands. The development is direct ; 

 the young are hatched as miniature adults. 



Anodonta (The Freshwater Mussel). 

 A Type of the Pelecypoda or Lamellibranchiata. 



Give a short Description of the Shell of Anodonht. 



The bivalve shell is rounded in front and bluntly pointed 

 behind. The two equal valves are connected together 

 by a dorsal and elastic hinge-ligament. The umbo, tlio 

 oldest part of the shell, is in front of the hinge. A suc- 

 cession of curved lines on the outside of the shell indicates 

 the growth, the extent between any two lines being that 

 of one year. The shell has three layers : the inner pearly 

 or nacreous layer is made by the surface of the mantle 

 which lines the shell ; the median prismatic limy layer 

 and the outer horny periostracum are formed by the thick- 

 ened edges of the mantle which are attached along the lower 

 inner borders of the valves. 



