50 COMPARATIVE ANATOMY CHAP. 



IV. Nerve Tissue. 



As muscle elements pure and simple may be imagined to arise out 

 of neuro-muscular cells by the gradual differentiation of most of the 

 protoplasm of one part of the cell into contractile substance, the 

 muscular function thus being brought to the front at the expense of 

 the other possible functions, so nerve elements may be produced by 

 the suppression of the contractile part of the cell and the further 

 differentiation of the neural portion. We can perhaps imagine that 

 the simple sensory cells of the body epithelium of the lower Metazoa 

 arose in this way, always presupposing that they remained in connection 



FIG. 50. Piece of a muscle lamella of the septum of an Actinian (Anthea cereus), with nerve 

 plexus, after 0. and R. Hertwig. m, Muscle fibres ; sz, sensory cell, with sensory hair ; gz, ganglion 

 cell. 



with the neighbouring contractile elements, either by simple contact 

 or by processes. In this way a stimulus received by the sensory cells 

 could be transmitted to the muscle cells. The sensory cells are 

 epithelial cells, which are generally distinguished by a delicate, usually 

 immobile, sensory hair, projecting outwards. Division of labour could 

 then go further. Single sensory or nerve cells of the most undifter- 

 entiated sort, found in contact with other sensory cells and muscle cells, 

 could give up their connection with the surface of the body and sink 

 into the lower part of the epithelium, thus playing the part of inter- 

 mediaries between sensory and muscle cells (Fig. 50), and transmit the 

 stimulus received by the former to the latter. Such cells are found in 

 many Ccelenterata. They are here already known as ganglion cells. 

 They possess processes by which they are connected with each other 

 and with the sensory cells and muscle fibres. They represent the 



