42 INVERTEBRATE PHYSIOLOGY 



planarians. Strong stimuli of either kind cause a reflex "escape" activity 

 pattern, with the "ghding" movement changing to a "crawHng" movement. 

 This reflex appears in an all-or-none manner, not showing differential 

 reflex behavior to weaker stimuli. 



It is a common observation that there is considerable variation among 

 individual worms to standard intensity stimuli. This feature appears to be 

 a commonplace of planarian behavior; not only do different individuals 

 vary, but the same animal shows opposite responses on different occasions. 

 Pearl suggested that this variation was due to varying "physiological or 

 tonic conditions" of the organism. He believed that the function of the 

 nervous system is to "preserve tonus," and claimed that if the "tonus" is 

 gone it is very hard to get positive reactions. This interpretation would give 

 an inhibitory or depressing role to the central nervous system. Against this 

 idea is the result of removal of the "cephalic ganglion" of the polyclad 

 Leptoplana (Hovey, 1929), which causes it to lose all tactile reflexes en- 

 tirely, becoming less sensitive rather than more sensitive to stimuli. How- 

 ever, the noticeable differences between triclad and polyclad nervous sys- 

 tems (Turner, 1946) may make this seeming contradiction meaningless. 



As in the case of annelids and mollusks, it seems clear that more must 

 be learned about the reflex physiology of these organisms before an attempt 

 can be made to explain their behavior patterns. How much of the be- 

 havior is dependent on the "cephalic ganglion" and how much is locally or 

 regionally autonomous? Furthermore, can the modification of stimulus 

 threshold by changes in the internal conditions, such as previous feeding 

 (Pearl, 1902) or lack of "symbiont" Hydra nematocysts (Kepner et al., 

 1938), be explained without recourse to postulating an integrating cen- 

 tral nervous system ? By demonstrating a conditioned reflex in Leptoplana, 

 Hovey (1929) apparently showed that the "brain" of these animals has 

 the necessary functional complexity. His localization of this conditioning 

 in the cephalic ganglia is not convincing, however. 



For three distinctly different sorts of reasons, the coelenterates differ 

 from the other phyla that we are considering here. First of all, they have no 

 well-defined nervous ganglia to exercise overriding control of their 

 nervous system.^ Secondly, we have reason to feel closer to an analysis of 

 their behavior in physiological terms than in the case of other phyla ; in 

 part, this is simply an expression of their apparent simplicity, but it is also 

 a tribute to the work of many famous biologists, among them being Ro- 



1 The marginal sense organs of scychozoans may be synaptic areas of a sort. In a 

 series of important papers, Horridge (1955a, b) presents evidence for the independence 

 of two conducting systems in hydromedusae, interconnected with polarized synapses. 

 He also suggests that ". . . the ring nerves, which have ganglion cells all along their 

 course..." cause inhibition of swimming activity during feeding activity (1956), 

 Thus the "ring nerves" may be acting as an integrating center in these jellyfish. 



