196 ZOOLOGY 
CLASS Gasteropoda. 
CHARACTERISTICS.— The GASTEROPODA have a foot which is in 
the main a crawling 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 Chaetoderma, 
another member of the subclass, is dioecious, in the former the 
generative cells escape by special ducts. In Meomenia 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.—Jn 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 ; tt 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 
