9 o 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



structure, no repetition of similar 

 parts in a linear series. Now it is 

 probable that the nervous system 

 controls the general structure of 

 the body. Thus while the nervous 

 system of vertebrates is a continu- 

 ous axis, and that of arthropods and 

 worms a string of ganglia running 

 through the body, in mollusks there 

 is no axial arrangement as in verte- 

 brates, nor a linear series of gan- 

 glion as in arthropods. 



General Plan of the Nervous Sys- 

 tem. — In these animals the nervous 

 system consists (i) of an oesophageal 

 collar, and (2) of ganglia irregular- 

 ly placed wherever important func- 

 tions, either nutritive or locomo- 

 tive, are situated. 



Examples. — 1. In bivalves (acephala), such as clams, 

 for instance, we have (Fig. 63) : (1) The oesophageal col- 

 lar, which presides over the mouth and head functions 

 (e. g., gathering of food and 

 whatever beginnings of intelli- 

 gence the creature may possess), 

 and also has general presidence 

 over conscious voluntary move- 

 ments, and therefore over other 

 ganglia; (2) a large visceral gan- 

 glion in the region of nutritive 

 and respiratory organs, to pre- 

 side over these; and (3) a loco- 

 ,.„. N| ..,, „ , motive or foot ganglion to con- 



an oyster : c, cephalic gan- trol locomotion. This is all. In 

 glion ; v } the visceral gan- . . 



glion. the oyster the nervous system is 



Fig. 63. — Nervous system 

 of a clam : eg, cephalic 

 ganglion ; pg, pedal 

 ganglion ; vg, visceral 

 ganglion. 



