PROTOZOA TO WORMS 



49 



plexus has thickened at this point into a little compact 

 mass of cells and fibres called a ganglion. And be- 

 cause this ganglion through- 

 out higher forms usually lies 

 over the oesophagus, it is 

 called the supra-oesophageal 

 ganglion. This is the first 

 faint and dim prophecy of a 

 brain, and it sends its nerves 

 to the front end of the body. 

 But there run from it to the 

 rear end of the body four to 

 eight nerve-cords, consisting 

 of bundles of nerve-threads 

 like our nerves, but overlaid 

 with a coating of ganglion 

 cells capable of originating 

 impulses. These cords are, 

 therefore, like the plexus from 

 which they have condensed, 

 both nerves and centres ; dif- 

 ferentiation has not gone so 

 far as at the front of the body. 

 Sense organs are still very 

 rudimentary. Special cells 

 of the skin have been modi- 

 fied into neuro - epithelial 

 cells, having sensory hairs 

 protruding from them and 

 nerve - fibrils running from 

 their bases. 



In a ver}' few turbellaria 

 we find otolith vesicles. 

 4 



OROS8-8B0TION OF TUBBELLABUH. 

 EAT3CHEK, FBOM JUTWA. 



e, external skin ; rm, lateral muscles ; 

 la and li, longitudinal mnecleB; 

 Ttidv, dorso-ventral muscles J pa, 

 parenchyma ; ft, testicle ; cm, ovi- 

 duct; at, yolk-gland; n, ventral 

 nerve ; i, gastro-vascular cavity. 



