COELENTERATA AND CTENOPHORA 



approach this level of specialization, functional elements constituting^ a "'nerv- 

 ous system" do occur and are somewhat more hitijhlv ors;ani/ed than those of 

 sponges (p. 280). 



If we recall the dehnition of responsiveness as the capacity, inherent in 

 living cells, of responding to stimuli or changes in the environment, it is 

 apparent that the foundation of behavior in the hydra, as in a vertebrate or 

 in a protozoan, is the responsiveness of cells. All the cells of the hvdra 

 are capable of responsiveness, but those of the sensory-neuro-muscular svstem 

 are specialized in this respect. Although experiments which have demon- 

 strated the nervous functions of larger animals cannot be performed with an 

 animal as small as the hydra, the functions of certain cells can be inferred 

 from their relationships and from our knowledge of the reactions of similar 

 cells in other animals. Reception of stimuli and establishment of nerve 

 impulses by the sensory cells, conduction and discharge of impulses bv the 

 neurons, and the resultant action of effectors such as contractile and secretorv 

 cells all occur in the hydra. 



If a "system" is defined as a "group of organs" performing some general 

 function, the hydra has no nervous system. Yet, we naturallv speak, as in the 

 foregoing discussion, of the "nervous system" in hydra. In general, the 

 nervous mechanism of coelenterates is a receptor-eflFector system, in contrast 

 with the receptor-adjustor-effector systems of higher animals (Fig. 10.6). 

 Further comparisons of the mechanisms of coordination in animals will be 

 deferred until after the nervous system of the earthworm has been described 

 in Chapter 14. The organization of this mechanism in annelids is inter- 

 mediate between that of coelenterates and that of vertebrates. 



Reproduction and Development. At certain seasons of the year, particu- 

 larly in autumn, hydras reproduce by syngamy, the union of gametes. The 

 testes are usually located on the distal half of the trunk, the ovaries near 

 the middle. Testes may appear first and ovaries later on the same animal, 

 or both may be present together. Animals in which the same individual 

 possesses both ovaries and testes are said to be hermaphroditic or monoecious. 



Cnidoblast 



Sensory cell 



Neurosensory 

 cell 



Muscle 

 fibril 



Supporting 

 lamella 



Fig. 10.6. Diagram of the relation.ships of the sensory-neuro-muscular system in the epidermis 

 of Hydra. One of the nematocysts, considered independent effectors, is included. The muscle 

 fibrils occupy the basal processes of large epithehomuscular cells. 



297 



