MOLLUSC A 197 



mantle developes a shell, which often increases the asymmetry 

 of the animal hy 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 tvith a mucous gland. 

 The Gasteropoda Anisopleura are subdivided into two 

 branches : Streptoneura (Prosobranchiata) and Euthyneura. 



BrancliA. STREPTONEURA. 



Characteristics. — The first branch comprises those Molluscs in 

 which the torsion has proceeded to such an extent that the 

 emus has become anterior, and the right gill and o^hradium 

 have crossed anteriorly to the left, whilst the left gill and 

 osphradium have come rozcnd posteriorly to the right. As a 

 consequence one limb of the visceral nerve loop is pulled over 

 the other and a figure of Q 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- 

 idium of one side has not usually taken place, 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 

 foot. 



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, Fissurella, 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 



