218 University of California Publications. IZoology 



met with also which show no signs of a zone of regeneration, 

 but possess only two pairs of perfect mesenteries. If they are 

 products of fission, as is probable, then in them regeneration 

 seems to be at a standstill. Such cases suggest the influence of 

 external factors; lack of food, for instance, might alone prevent 

 the return to the parent condition which might otherwise have 

 occurred. 



3. The third method of non-sexual reproduction in 8. davisi 

 may be described as aboral-oral division by constriction. This is 

 the least conspicuous method of the three, occurring so rarely 

 that we have never seen the completion of the process iu a 

 normal individual. In consequence, we cannot demonstrate its 

 normal occurrence, but are strongly inclined, from indirect evi- 

 dence, to believe that it does actually play some part, though a 

 very small one, in the propagation of the species. 



a b 



Fi<l. 7, a, b, c. A series of polyps which may represent different stages of 

 aboral-ora] fission by constriction. 



In the first place, cases that appear to represent stages in the 

 process have been found which can be arranged in a progressive 

 series (Fig. 7 a, b, c) . Case C might have risen as the result of 

 an accidental tear through the foot disk, a condition we have 

 mentioned as sometimes occurring in Metridium, where it has no 

 connection with normal methods of non-sexual reproduction. 

 Against this view, results of experiments to be described below 

 maybe brought, indicating that a tear of such proportions would 

 probably initiate a fission that would reach a speedy completion. 

 We have no facts to indicate that c represents a double monster 

 sprung from an abnormal embryo, and do not favor such a view. 



On the other hand, the condition represented in a has been 

 met with many times as a resting condition, though identical 



