354 NERVOUS SYSTEM OF MOLLUSKS. 



side. Of these, one passes through the ganglionic masses cc, to 

 communicate with the ganglion B, which is the one connected 

 with the respiratory movements. The others are distributed 

 with the branches of the ganglia cc, the function of which is 

 double ; for one set of branches from each is distributed to 

 the mantle in general, every part of which (in these shell-less 

 Mollusks) is capable of contracting and giving motion to the 

 body; whilst another set is distributed to that thick and 

 fleshy part of it which is called its foot, and on which the 

 animal crawls ( 107). There is another ganglion, D, lying 

 in front of the cephalic ganglion, and also receiving branches 

 of communication from it ; this ganglion is specially connected 

 with the actions of mastication and swallowing, and is called 

 the pharyngeal ganglion. 



439. Thus we see that the cephalic ganglion sends branches 

 to all the other ganglia, though these having different functions, 

 do not communicate with each other; and thus every part 

 has two sets of nervous connexions, one with the cephalic 

 ganglia, and the other with its own ganglion. By the former, 

 the animal becomes conscious of impressions made upon it, 

 these impressions being converted in the cephalic ganglia into 

 sensations ; and the influence of its conscious power is exerted 

 through them upon the several parts of its body, causing 

 spontaneous motion. By the latter are produced those reflex 

 actions of the several organs, which do not require sensation, 

 but which depend upon the simple conveyance of an im- 

 pression to the ganglion, and the transmission of the resultant 

 motor impulse from it to the muscles supplied by its nerves. 

 A small part only of the Nervous System of Mollusks 

 ministers to the general movements of the body; and this 

 corresponds with what has been elsewhere stated ( 107) of 

 the inertness which is their general characteristic, and of the 

 small amount of muscular structure which they possess. 



440. On the other hand, in the ARTICULATED classes, in 

 which the apparatus of movement is so highly developed, and 

 whose actions are so energetic, we find the Nervous System 

 almost entirely subservient to this function. Its usual form 

 has been already described ( 94) as a chain of ganglia con- 

 nected by a double cord, which commences in the head, and 

 passes backwards through the body. In general, we find a 

 ganglion (or rather a pair of ganglia united on the middle 



