GENERAL ZOOLOGY 



and have processes, presumably involved in sensory reception, extending to 

 the outer surface of the epidermis. Nerve cells and receptors are also found 

 in the gastrodermis, although in smaller numbers. The epidermal and gas- 

 trodermal elements are generally considered as forming two networks, 

 apparently rather sparsely interconnected by neurons traversing the support- 

 ing lamella. The richer innervation of the epidermal longitudinal muscula- 

 ture is reflected in the greater degree of coordination of its activities, as 

 compared with those of the gastrodermal circular musculature. 



The nature of the coelenterate sensory-neuro-muscular system has been the 

 subject of much disagreement among zoologists. The controversy has 

 centered about the question whether the network of neurons represents a 

 morphological continuum, composed of cells whose processes are actually 

 physically continuous with each other, or whether the extremities of adjacent 

 neurons are merely in synaptic contact as in the nervous systems of all higher 

 animals. Reliable evidence bearing upon this question is difficult to obtain 

 in an animal as small as the hydra, but it is probably safe, where necessary, 

 to extend to the hydra the results of studies on larger coelenterates. Modern 

 research has demonstrated almost beyond question that the network is not 

 continuous, and that nerve impulses traveling through the system must pass 

 from cell to cell across a discontinuity, or synapse (pp. 96-100). Al- 

 though the processes of adjacent neurons may be in contact or actually 

 intertwine with each other, there seems to be no protoplasmic continuity 

 between them. If this is true, it probably follows that the passage of a 

 nerve impulse across a synapse in coelenterates, as elsewhere in the Animal 

 Kingdom, involves the secretion by the neuron of a chemical mediator which 

 e.xcites the adjacent neuron. 



However this system may thus seem to agree, in its basic mechanism, with 

 those of more advanced animals, there are two fundamental and interrelated 

 differences which emphasize its primitive nature. One is the fact that this is 

 a diffuse, net-like system, with no indication of the formation of ganglia, the 

 aggregations of nerve cells so characteristic of the nervous systems of all 

 higher forms. The other is the fact that the neurons and synapses of coelen- 

 terates conduct impulses in any direction with apparently equal facility; in 

 other words, there is no indication here of polarization within the nervous 

 system. In higher animals conduction within one neuron may occur in either 

 direction, but the neuron is polarized in the sense that the impulse can be 

 propagated across a synapse at only one end. This is the basis of the one- 

 way transmission characteristic of the nervous systems of animals more ad- 

 vanced than coelenterates. In the hydra and its relatives a nerve impulse 

 must be conducted over a long pathway, wandering along the branching proc- 

 esses of widely spread neurons, possibly exciting inappropriate connections 

 along the way. In contrast to this, a ganglionated system, with polarized 

 synapses, offers the special advantages of contiguity of nerve cell bodies inter- 

 connected by short processes, promoting the channelization of impulses with 

 greater efficiency along definite pathways. Although the coelenterates do not 



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