STRUCTURE AND FUNCTION IN INVERTEBRATES 



Stimulation - 



Neuron 



Response 



Receptor 



Neuron 



Response 



Effector 



Stimulation 



Response 



Central nervous system 



Receptors 

 Sense organs 



Muscles and glands 



Fig. 17.7. Mechanisms of nervous integration among invertebrates; diagrammatic. A, receptor- 

 effector svstem of a calcareous sponge. B, receptor-effector system of Coelenterata. C\ 

 receptor-adjustor-effector system, as in Annelida and all the more complex invertebrates. 

 {A, redrawn from O. Tuzet, R. Loubatieres,. and M. Pavans de Ceccaty, 1952, Comptes rendiis 

 de rAcademie des Sciences, vol. 234.) 



are shown schematically in Figure 17.7. The interconnections between the 

 cellular elements in this system seem to be synaptic, as in higher forms, 

 rather than continuous protoplasmic bridges, as was once believed. The 

 nematocysts appear to function as independent effectors. In many jellyfishes 

 there are quite well-developed sense organs, such as the eyes and organs of 

 equilibration. No concentrations of nerve cell bodies to form ganglia occur in 

 coelenterates, however, and there are no functional indications of the presence 

 of a central nervous system, with adjustment of impulses and directed reflex 

 activity. The nervous system of coelenterates may be characterized, there- 

 fore, as a receptor-effector system, corresponding functionallv to that of 

 sponges but more highly organized. 



The nervous system of annelids is representative in general of that found 

 in most higher invertebrates. Specifically, as in the earthworm, the receptor 

 elements are neurosensory cells in the epidermis. Each of these cells has a 

 principal process or axon, extending as a nerve fiber which enters the ventral 



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