MOLLUSC A 203 



duct. The left nephridium, which was in Patella smaller 



than the right, and the left ctenidium and osphradium, are 



retained. The anus lies on the right of the neck of tJie 



animal. The Azygdbranehiata are dioecious, and the males 



are often furnished with a large penis. 



The great majority of Azygobranchiata are adapted for 



creeping at the bottom of the sea (Reptantia), and for this 



purpose have a large muscular foot with a flat sole. They 



are often spoken of as sea-snails ; the shell which encloses and 



protects the visceral hump is usually coiled. A large gland 



is not unfrequently found lying alongside the rectum ; in the 



genera Murex and Purpura its secretion turns purple when 



exposed to the light, and it was in ancient times used as a 



dye. The posterior surface of the foot in some species bears 



a calcareous or horny plate, the operculum, which serves to 



close the mouth of the shell when the animal is retracted. 



The foot itself shows a tendency to break up into three 



portions : anteriorly the propodium, in the middle the meso- 



podium, and posteriorly the metapodium, which bears the 



operculum. 



This group includes, amongst many others, Buccinum, the 

 whelk, and Littorina, the periwinkle. Paludina, the river-snail, 

 and Valvata are freshwater members of the group, and so is 

 the terrestrial Cyclostoma, which has no gill, the mantle 

 chamber having become a respiratory organ. It lives in 

 damp places. Untoeoncha mirabilis lives parasitically in the 

 body of Synapta digitata ; this is exceptional, as parasitism is 

 very rare amongst the MoUusca. 



A few Azygobranchiata have become modified in con- 

 nection with a pelagic mode of life. These form the section 

 Natantia, also known as the Heteropoda. As in other 

 pelagic organisms, their tissues have become wonderfully 

 transparent, and of a gelatinous consistency. The foot has 

 become a swimming organ. Its division into pro-, meso-, and 

 meta-podium is well marked. Atlanta has a coiled transparent 

 shell, into which the body and foot may be withdrawn, and 

 the metapodium carries an operculum. In Garinaria the 

 foot is by far the largest part of the animal, and the relatively 

 small visceral hump is covered by a small hyaline shell. In 



