THE PHYLA COELENTERATA AND CTENOPHORA 



187 



When a jellyfish is not digesting food the mouth usually remains 

 open and fresh sea water is circulated through the gastrovascular system 

 by the ciliated tracts. Hence, most of the time all of the tissues are in 

 direct contact with sea water, facilitating a direct exchange of gases and 

 waste products by diffusion. In all probability the water in the gastro- 

 vascular system contains enough oxygen to supply the gastrodermis cells 

 while the mouth is closed during digestion. 



63. Gonionemus: Nervous System 



Classically the nervous system of the coelenterate is described as a 

 nerve net, a diffuse network of neurons each with several processes that 

 synapse with those of other neurons. The system is distinguished from 

 those of higher organisms by the transmission of impulses across synapses 

 in either direction, rather than in one direction only. The concept of a 

 generalized nerve net, however, does not adequately explain the specific 

 coordinated behavior of the medusa. Detailed work has shown that the 

 system is not this simple. On the upper or exumbrellar surface of 

 the medusa the neurons are sparse and their arrangement is indeed that 

 of a simple net. At the margin, however, the nerve cells are concentrated 

 to form circular fiber tracts, the nerve ring. On the subumbrellar surface 

 the nerve fibers are arranged radially, extending from the margin to- 

 ward the center with few if any circular fibers. 



The nerve ring of Gonione?7iiis is double (Fig. 10.6), with rings 

 above and below the line where the velum is attached. The lower ring 

 is primarily motor in function and sends fibers to the muscles. The upper 

 ring is primarily sensory and integrates the information coming in from 



Edc umbr ellctr 

 epithelium: 



Sensor" 37 

 epithe-liiim. 



Uppers 

 nerve iri-n.^ 



Loureip 

 nerve rin^ 



G a.strodermiS 



Subuiritrellar 

 epit Inel i um. 



Figure 10.6. Section through the margin of the bell in Gonionemus, showing the 

 nerve ring. It lies embedded in the double layer of epiderrnis at the base of the velum, 

 (After Hyman.) 



