STEUCTURE OP MOLLUSCA. 



115 



b b; to this cavity, water has access by means of a wide slit, 

 of which the edge, /, of the mantle forms one part of the 

 border, whilst at d is seen a fringed membrane that forms 

 another part. At c is seen the heart, which receives the 

 blood from the gills by v b, the branchial vein, and then 

 transmits it to the body generally ; at e, far up in the spire, 

 are the stomach and liver ; at cc, 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 nervous system of the Mollusea 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- 

 tained in the head of Insects ; but 

 of the latter, one only is connected 

 with the foot or organ of motion, 

 the remainder having for their 

 function to regulate the action of 

 the gills, and to perform other 

 movements connected with the 

 operations of nutrition. In fig. 56 

 is represented one of the simpler 

 forms of this nervous system, 

 that of the Pecten or Scallop-shell; 

 A A are the ganglia near the mouth, 

 from which the organs of sense 

 are supplied; B is the ganglion connected with the gills; 

 and c is that from which power is given to the foot. The 



i2 



Fig. 56. NERVOUS SYSTEM OF 

 PECTEN. 



