444 



GENERAL CONSIDERATIONS 



coelenterates nerve cells are met for the first time, and these exhibit more 

 or less variety. 



Generally speaking, in the coelenterates are found certain ectodermal 

 or entodermal cells which receive stimuli and which may be called sense 



cells or receptor cells. These are connected to nerve 

 cells lying deeper in the ectoderm or entoderm, which 

 with their connecting and conducting fibers form a 

 nerve net. This net is structurally continuous and 

 is intimately connected to the contractile fibers. 

 There is a tendency for these nerve cells to concen- 

 trate in one ring about the hypostome and in another 

 about the basal disc (Fig. 295). In the flatworms 

 this tendency toward centralization results in the 

 development of central ganglia below the eyespots and 

 a ganglionic cord on each side, and central and periph- 

 eral portions may be distinguished in a nervous system 

 (Fig. 72C). This also leads to a type of reflex action. 

 A typical reflex act has been described in connec- 

 tion with the earthworm (Sec. 279). This results 

 from the development of a synaptic nervous system. 

 The unit of such a nervous system is the neuron, or 

 nerve cell, and neurons are related to each other 

 through synapses in which their fibers come into 

 intimate contact. At first, reception of stimuli is by 

 neurons lying upon the surface and called receptor 

 neurons. As the nervous system becomes more highly 

 developed the neurons are withdrawn from the surface and certain epithe- 

 lial cells receive the stimulus and may be associated in groups in sense 

 organs or receptors. These modified epithelial cells receive the stimulus 

 and pass it over to the dendrites of the sensory neurons, which are lodged 

 in ganglia within the body. In the vertebrates such neurons are found in 

 the dorsal root ganglia of the spinal and cranial nerves (Fig. 218). In all 

 cases, however, neurons are ectodermal in origin and are developed upon 

 the surface early in embryonic life, later to sink deeper in the body and 

 reach their ultimate location. 



The types of nervous systems may be named as follows: 

 1. Reticular Nervous System. — This has also been called a diffuse 

 nervous system. In its primitive form such a system consists of a nerve 

 net; this is found only in the coelenterates. In some coelenterates, 

 ctenophores, and echinoderms ganglion cells are added. In the flat- 

 worms and the phyla which follow them ganglia are present. Nerve nets 

 persist in all higher forms and are found even in the walls of the blood 

 vessels and alimentary canal in vertebrates. 



Fig. 295.— a 

 young hydra show- 

 ing the nerve net, 

 which tends to be 

 condensed in one 

 zone about the base 

 and another about 

 the hypostome. 

 (From Hadzi, Arhei- 

 ten aus der Zoolog.- 

 Institut der Univer- 

 sitcit Wien, vol. 17.) 



