MORPHOLOGY OF ANIMALS 



(Fig. 22, D, E). A nerve cell with all of its outgrowths 

 is called a neuron. In practically all Metazoa these cells 

 are derived from ectoderm, and in a good many animals 

 the sense organs and entire nervous system remain 

 throughout life a part of the superficial epithelium which 

 covers the body (Coelenterata, Chaetognatha, certain An- 

 nelida, Molluscoidea, many Echinodermata, Balanoglos- 

 sus) ; such a nervous system is said to be epithelial (Fig. 

 22, A). In all other Metazoa the nervous system, though 

 formed from epithelium, separates from it in the process 

 of developmicnt, so that brain, ganglia, and nerve trunks 

 come to lie some distance from the surface of the body; this 

 is known as an epitheliogenous nervous system (Fig. 22, 

 B, C) . In addition to the two classes just mentioned, which 

 are based on the relations of the nerve cells to the body 

 layers, four types of nervous system are found among 

 Metazoa which are based upon the relations of the nerve 

 cells to one another; these are (i) the diffuse type, (2) 

 the linear type, (3) the ganglionic type, and (4) the 

 tubular type. 



(1) A diffuse nervous system^ consisting of nerve cells 

 and fibres scattered throughout the superficial epithelium, 

 is the simplest type known and is found among such 

 animals as hydroids and sea anemones; the nerve cells are 

 here connected together by means of fibres into a nerve 

 plexus. 



(2) The next step in increasing complexity is repre- 

 sented by a linear nervous system found in jellyfishes, 

 echinoderms, fiat- worms (Fig. 11, C) ; here many nerve 

 cells and fibres are aggregated into definite lines or strands, 



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