450 - Multicellular Animals, Especially Man 



NUCLEUS 



NUCLEUS- 

 MUSCLE PROCESS 



EPITHELIOMUSCLE CELL MYOFIBRILS 

 NUCLEUS 



NERVE FIBERS 

 SENSORY CELL 



NERVE FIBERS 



NETTLE CELL 

 UNDISCHARGED 



Fig. 25-4. Specialized responsive cells of Hydra. 



NETTLE CELL 

 AFTER DISCHARGE 



nerve fibers of higher animals, since they re- 

 ceive impulses from specialized receptor cells; 

 but other branches of the net are motor 

 fibers, since they transmit impulses to the 

 epitheliomuscle cells of the ectoderm and 

 endoderm. 



The nerve net of Hydra is composed of 

 primitive nerve cells, called protoneurons 

 (Fig. 25-4), which differ from the neurons of 

 higher animals. Protoneurons may not be 

 discrete cells. In fact the whole nerve net 

 may represent a syncytium, in which the 

 protoneurons are in protoplasmic continuity 

 with each other. 1 Moreover, protoneurons 

 tend to send out fibers in many directions 

 from the centron (p. 453), rather than along 

 one main linear pathway of contraction, as 

 is the case with the neurons of higher ani- 

 mals (p. 453). 



In Hydra, impulses from the receptors in 

 any part of the body tend to spread rather 



1 Recent electronmicrographic studies indicate that 

 at least some synapses are present in the nerve net. 



indiscriminately in all directions throughout 

 the nerve net. In fact, any strong stimulus 

 tends to produce a very generalized response, 

 such as a contraction of the hydra as a whole, 

 since a large proportion of the epithelio- 

 muscle cells are thrown into action more or 

 less simultaneously. 



There are, however, some factors that tend 

 to localize responses, even in the hydra's 

 primitive nervous system. Due to the elon- 

 gate shape of the tentacles, the nerve net in 

 these organs runs mainly in a lengthwise 

 direction. This arrangement tends to orient 

 the flow of impulses to and from the tenta- 

 cles giving the tentacles some capacity for 

 reacting on an individual basis. Moreover, 

 protoneurons display a relatively long re- 

 fractory period, so that recently excited parts 

 of the nerve net are not immediately capable 

 of propagating new excitations. Probably 

 there are other factors, still unknown, that 

 tend to localize the responses of Hydra more 

 definitely. 



