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 verv few turbellaria 



> 



we find otolith vesicles. 

 4 



6. CROSS-SECTION OF TURBELLARIAN. 

 HATSCHEK, FROM JIJIMA. 



e. external skin ; rm, lateral muscles ; 

 la and li, longitudinal muscles ; 

 mdv, dorso-ventral muscles ; pa, 

 parenchyma ; h, testicle ; ov, ovi- 

 duct ; dt, yolk-gland ; n, ventral 

 nerve ; ?, gastro-vascular cavity. 



