86 



GENERAL PRINCIPLES OF ZOOLOGY 



system. In the ganglia (the nervous centres of the last three groups) 

 the ganglion cell usually gives rise to a single strong process, which, how- 

 ever, is richly provided with lateral branches or dendrites (fig. 77). The 

 medullary sheath and sheath of Schwann are usually absent in inverte- 

 brates even in the peripheral nerves. On the other hand, the true con- 

 ducting elements, the nerve fibrilloe, have been seen in invertebrate nerve- 

 fibres, and have been followed into the ganglion cell in which the afferent 

 and efferent fibrilke are united in a lattice-like manner. 



FIG. 57. Ganglion cells of an actinian. 



A ganglion cell with its dendrites and neurites (the latter in large animals 

 may be several feet long, since they extend from the central nervous system to 

 the muscles) form a physiological unit, called a neurotic. The advocates of the 

 'neurone theory' maintain that the processes of two neurones do not anastomose 

 and that there is no continuity between them; they approach each other so closely 

 that the nervous impulse may jump from one to another like an electric spark. 

 The opponents of the theory assert that there is real continuity as seems to be 

 certainly shown with the giant ganglion cells of the nematodes. A second 

 disputed point is that the neurone theory holds that all processes, even the long- 

 est neurite, grows from the ganglion cell and throughout its length is a product 

 of it. The opponents claim that the neurites which compose the peripheral 

 nerve fibres are formed by special nerve-forming cells (neuroblasts) and these 

 contribute to the length of the neurite starting from a ganglion cell. In many 

 invertebrates (Hirudinei) such neuroblasts persist in the course of the peripheral 

 nerves. Also the peripheral system of invertebrates can consist of a network of 

 anastomosing neuroblasts (gang ionic plexus of medusae and chaetognaths). 

 When we recall that the primitive ccelenterates (hydroid polyps) lack a central 

 nervous organ and the whole nervous system is only a plexus of similar nerve 

 cells, we may suppose that from this primitive condition the more highly devel- 

 oped forms of nervous systems have developed through division of labor, cells in 

 a suitable position giving rise to the central system, while the remaining cells of 

 the plexus form the conducting tracts. Recent experiments in growing nerve 

 cells in nutrient fluids seem to support the view that the axons are exclusively 

 products of the cell. 



