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 Cephaloptiora, which are provide^ 

 with eyes and powerful acti}e tentacles. The two 

 most important phenomena observable jn the charac- 

 ters of the nervous system ,of this group are th,e fusion 

 of the primitively separate ganglionj.c masses, and the 

 twisting undergone by the nerve cords of soine of the 

 Gastropoda. The former attains its most marfceql 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, a#d from the left to the right-hand side. 



From the ganglionic masses and from the cords that 

 connect; hem together m the way that }ias now been 

 described, nerves are giyen 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 qf nerve fibres and cells ; as the cells became 

 gradually aggregated intp plefinjte mass^s, the n,erves 

 that were given off from then} became likewise arranged 

 in a defjnjte and regular fashion } and took on definite 

 duties, and functions. Those nerves jthat pass to 

 muscles may be spoken of as the motor pr efferent 

 nerves, those that end in sensory organs, whether 

 general tactile organs or organs of more espepia} sense, 

 as sensory or afferent nerves ; that is tp 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 



