MOLLUSCA 197 
mantle developes a shell, which often increases the asymmetry 
of the animal by being spirally coiled. This shell is often 
capacious enough to shelter the whole animal, thus forming a 
kind of house into which the animal can withdraw. The 
foot is usually provided with a mucous gland. 
The Gasteropoda Anisopleura are subdivided into two 
branches: Streptoneura (Prosobranchiata) and Euthyneura. 
Branch A. STREPTONEURA. 
CHARACTERISTICS.— The first branch comprises those Molluses in 
which the torsion has proceeded to such an extent that the 
anus has become anterior, and the right gill and osphradium 
have crossed anteriorly to the left, whilst the left gill and 
osphradium have come round posteriorly to the right. As a 
consequence one limb of the visceral nerve loop is pulled over 
the other and a figure of & is produced. 
This branch includes two orders: Zygobranchiata and 
Azygobranchiata. 
Order 1. ZYGOBRANCHIATA. 
CHARACTERISTICS.— The first order includes all those forms in 
which, although the torsion is complete, so as to bring the anus 
near to the anterior median line, the atrophy of the cten- 
udium of one side has not usually taken lace, and the 
generative cells leave the body through one of the nephridia 
which still retains its renal function. No accessory generative 
organs occur, and the visceral hump is coextensive with the 
Soot. 
This group includes three families. The best-known genera 
are Haliotis, known as the Ormer in the Channel Islands, where 
it forms an article of diet, Fisswrella, and Patella or the 
limpet. 
Patella vulgata, the common limpet, is protected by a 
conical dome-shaped shell, whose average length is about two 
inches. The edges of the shell are not quite smooth, and their 
inequalities generally correspond closely with those of the 
rock upon which the animal is situated. Limpets are usually 
found between the tide-marks, and if they wander away from 
the spot on which they usually occur when covered by the 
