STRUCTURE OF MOLLUSCA. 115 
66; to this cavity, water has access by means of a wide slit, 
of which the edge, 7, of the mantle forms one part of the 
border, whilst at d is seen a fringed membrane that forms 
another part. At ¢ is seen the heart, which receives the 
blood from the gills by v6, the branchial vein, and then 
transmits it to the body generally ; at e, far up in the spire, 
are the stomach and liver ; at a, the anal orifice of the intes- 
tine within the branchial cavity, and at ov the oviduct, which 
opens in the same situation. 
109. Thus it is seen that,—whilst the body of an Articu- 
lated animal may be compared to that of a man in whom the 
apparatus of nutrition (contained in the chest and abdomen) 
is of the smallest possible size, but whose limbs are strong, 
and his movements agile,—the body of a Mollusk resembles 
that of a man “ whose god is his belly,” his digestive appa- 
ratus becoming enormously developed, whilst his limbs are 
feeble, and his movements heavy. Such varieties, in a greater 
or less degree, are continually presenting themselves to our 
notice. 
110. The neryous system of the Mollusca generally consists 
of a single ganglion or pair of ganglia, which are placed in the 
head, or (when that is deficient) in the neighbourhood of the 
mouth ; and of two or more separate ganglia, which are found 
in different parts of the body, and are connected with the 
preceding by nervous cords. The 
former correspond to those con- 
ied in the head of Insecta; bub.) ey 
of the latter, one only is connected , 
_ with the foot or organ of motion, 4 
the remainder having for their iY 
function to regulate the action of 
the gills, and to perform other PbS : 
movements connected with the 4 
"operations of nutrition. In fig. 56 7) { 
¥, 
is represented one of the simpler (\ 
forms of this nervous system,— 4 
that of the Pecten or Scallop-shell ; 
AaAare the ganglia near the mouth, Fig. 56.—Nearvous Sxstem or 
from which the organs of sense Se a 
are supplied; B is the ganglion connected with the gills; 
and ¢ is that from which power is given to the foot. The 
