38 THE WHENCE AND THE WHITHER OF MAN 



behave in this respect much like a colony of amoebae. 

 The cells of both layers have at their bases long mus- 

 cular fibrils, those of the ectodermal cells running 

 longitudinally, those of the entoderni transversely. 

 The animal can thus contract its body in both direc- 

 tions, or, if the body contain water and the transverse 

 muscles are contracted, the pressure of the water length- 

 ens the body and tends to extend the tentacles. 



On the outside of the elastic membrane, just be- 

 neath the ectoderm, is a plexus or cobweb of nervous 

 cells and fibrils. As in every nervous system, three 

 elements are here to be found. 1. An afferent or sen- 

 sory nerve-fibril, which under adequate stimulus is set 

 in vibration by some cell of the epidermis or ectoderm, 

 which is therefore called a sensory cell. 2. A central 

 or ganglion cell, which receives the sensory impulse, 

 translates it into consciousness, and is the seat of 

 whatever powers of perception, thought, or will the 

 animal possesses. This also gives rise to the effer- 

 ent or motor impulses, which are conveyed by (3) a 

 motor fibril to the corresponding muscle, exciting its 

 contraction. But there are also nerve-fibrils connect- 

 ing the different ganglion cells, so that they may act in 

 unison. In the higher animals we shall find these cen- 

 tral or ganglion cells condensed in one or a few masses 

 or ganglia. But here they are scattered over the whole 

 surface of the elastic supporting membrane. 



The reproductive organs for the production of eggs 

 and spermatozoa form little protuberances on the out- 

 side of the body below the tentacles. But hydra re- 

 produces mostly by budding ; new individuals grow- 

 ing out of the side of the old one, like branches from 

 the trunk of a tree, but afterward breaking free and 



