ANODONTA. - 269 



] fence we have — Ovotestis 



. I 



genital duct 

 albuAien gland 

 common duct 



i I I. <? 



oviduct 

 vas deferens | 



I spermatheca and duct 



flagellum | 



I mucus glands 



penis I 



vagina 

 dart -sac 



The functions of these organs, so far as known, ^re : — 

 The snail is a protandric hermaphrodite, i,e.^ the male organs 

 Decome mature first and the female after. The ovotestis gives rise to 

 spermatozoa, which pass down the genital duct, the common duct and 

 the vas deferens into the flagellum. Here they are aggregated into a 

 rod- like mass, the spermophore. During this process darts are secreted 

 in the dart-sac and forcibly ejected from the genital pore into the skin of 

 other snails. This is followed by copulation, when the sperms are intro- 

 duced by the penis of one snail into the base of the spermathecal duct 

 of another. They pass up into the spermatheca and are there retained. 

 The ovotestis next produces eggs which pass down the genital duct 

 to the head of the common duct. The sperms then leave <^he sperma- 

 theca, make their way down the spermathecal duct and ba k again up 

 the oviduct and common duct, at the upper end of which they fertilise 

 the eggs. Albumen is then added to the eggs from the albumen gland, 

 and they pass down to the vagina. Here they are covered with mucus 

 from the mucus glands and are discharged to the exteriof. In some 

 species they are contained in calcareous shells. The eggs* are laid in 

 damp earth and the development is embryonic, the young newly-hatched 

 snail differing little from its parent. 



II.— ANODONTA. 



Phylum - - - Mollusca (p. 282). 



Class - - - Lamellibranchiata (p. 284). 



Anodonta cygnea* (the freshwater mussel) is a con- 



_ venient example of the large and important class 



of Lamellibranchiata, or bivalve molluscs. A 



full-grown individual may be as long as five inches. I'he 



* This description also applies to A nodonta anatina. 



