196 ZOOLOGY 



Class Gasteropoda. 



Characteristics. — The Gasteropoda Tmm a foot which is in 

 the main a cravMng organ, it is simple, median, and has a 

 broad flat surface. The foot is often divisible into three 

 divisions, termed the pro-, meso-, and meta-podium. 

 The Gasteropoda are divided into two sub-classes : 



i. Gasteropoda Isopleura. 



Characteristics. — The Gasteropoda Isopleura retain the primi- 

 tive bilateral symmetry of the group. The body is elongated, 

 the mouth anterior and the anus posterior. The viscera 

 generally are paired and bilaterally symmetrical. 

 This subclass includes six genera, which are distributed 

 amongst three orders. The best-known genus is Chiton, in 

 which the shell is metamerically divided into eight parts. The 

 gills or ctenidia are also metamerically repeated to the number 

 of sixteen or more, and at the base of each is a patch of 

 olfactory epithelium, the osphradium. Chiton, like Chcoetoderma, 

 another member of the subclass, is dioecious, in the former the 

 generative cells escape by special ducts. In Neomenia and 

 Chaetoderma, however, they leave the body by means of the 

 nephridia. 



The nerve ganglia are not very markedly developed, but 

 ganglion cells are scattered all along the well-defined nerve- 

 trunks. In some Chitons, eyes furnished with a lens, retina, 

 cornea, etc., have been described as existing on the shell 

 plates. 



ii. Gasteropoda Anisopleura. 



Characteristics. — In the members of this subdivision the head 

 and the foot have retained a bilateral symmetry, but the 

 visceral hump with its included organs has undergone a twist 

 which has resulted in rotating the anus and posterior part of 

 the viscera to the right. The angle through which the anus 

 has been twisted varies in different groups ; it may be as much 

 as 180°, and in this case the anus lies above the middle line 

 of the neck. One of the ctenidia is usually atrophied, and- 

 one of the nephridia specialised as a generative duct. The 



