Chap. XII.] MOTOR AND SENSORY NERVES. 411 



the Lamellibranchiata (Acephala) are always com- 

 paratively small, in consequence of the reduction of 

 the head of these Molluscs, they are' always much 

 larger in the Ceplialophora, which are provided 

 with eyes and powerful tactile tentacles, The two 

 most important phenomena observable in the charac- 

 ters of the nervous system of this group are the fusion 

 of the primitively separate gangiionic masses, and the 

 twisting undergone by the nerve cords of some of the 

 Gastropoda. The former attains its most marked de- 

 velopment in the Cephalopoda, where the pedal fuse 

 with the visceral ganglia, and are closely approxi- 

 mated to the cerebral mass ; the latter, which may be 

 seen in the limpet (Patella), or the river-snail (Palu- 

 dina), results in the nerves which connect the cerebral 

 with the visceral ganglia passing from the right to 

 the left, and from the left to the right-hand side. 



From the gangiionic masses and from the cords that 

 connect them together in the way that has now been 

 described, nerves are given off to various parts of the 

 body. We have already seen that in the lower forms 

 the whole of the body is invested in a superficial 

 plexus of nerve fibres and cells ; as the cells became 

 gradually aggregated into definite masses, the nerves 

 that were given off from them became likewise arranged 

 in a definite and regular fashion, and took on definite 

 duties and functions. Those nerves that pass to 

 muscles may be spoken of as the motor or efferent 

 nerves, those that end in sensory organs, whether 

 general tactile organs or organs of more especial sense, 

 as sensory or afferent nerves ; that is to say, they 

 bring messages to the central system, while the efferent 

 nerves carry messages away. The size and number of 

 these nerves depend, therefore, primarily on the size 

 of the parts to which they are distributed. Their 

 general arrangement may be well seen in a segmented 

 animal ; putting aside for a moment the nerves given 



