210 



ELEMENTS OF BIOLOGY 



are not clearly differentiated, although a primitive receptor-effector 

 mechanism exists in the functions o£ some of the ectodermal cells. 

 The most primitive of the nervous mechanisms of Metazoa occurs 

 in the members of the phylum Coelenterata, particularly in the 

 Hydrozoa. In these forms, hydra for instance, some of the ecto- 

 dermal cells are linked together by 

 means of protoplasmic processes, to 

 form a sort of nerve net (Fig. 139). 

 The contacts between the processes 

 that form the net are said to be direct, 

 the cytoplasms being fused. The net 

 is therefore a continuous cytoplasmic 

 structure and a stimulus set up in one 

 region may be transferred elsewhere 

 without crossing a cell boundary. 

 Some of the processes of the net com- 

 municate with the contractile cells of 

 the ectoderm; the nerve net together 

 with these epithelio-muscular cells con- 

 stitutes the most primitive type of 

 nerve-muscle operating unit. 



The Neuron. The nerve cell of 

 other Metazoa in general is so mark- 

 edly different in both structure and 

 function from other types of cells that 

 it is known by a separate name, neuron. A neuron consists essen- 

 tially of a cell body containing a centrally placed nucleus and 

 equipped with one or more long extensions of the cell substance 

 which may be referred to as nerve fibres. If the fibre is one that 

 conducts stimuli toward the cell body, afferent, it is known as a 

 dendrite. If the fibre is efferent, that is, conducts stimuli away from 

 the cell body, it is designated as an axone. In some cases the den- 

 drites and axones may be quite long; for instance, the cell bodies of 



Fig. 139. — Diagram illustrating 

 the distribution of the nerve net 

 in the ectoderm of a hvdra. 



