MOLLUSCA 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 the 
animal. The Azygobranchiata 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 Zittorina, 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. ntoconcha mirabilis lives parasitically in the 
body of Synapta digitata; this is exceptional, as parasitism is 
very rare amongst the Mollusca. 
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shell, into which the body and foot may be withdrawn, and 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 Carinaria 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 
