THE AXIAL GRADIENTS IN HYDROZOA. 209 



sort in a late bud and Fig. 76 another in a full grown animal. 

 Numerous other similar cases have been observed. It is of course 

 possible that both the unsymmetriral contraction and the early 

 disintegration indicate a local injury, although none was observed 

 in these and other similar cases. Visible injuries to the body wall 

 do become centers of early disintegration, undoubtedly in conse- 

 quence of the excitation produced by the injury, and they may 

 also bring about local contraction, though in many cases they 

 do not. While local injury cannot therefore be entirely excluded 

 as a factor in such centers of early disintegration, it seems evident 

 that the early disintegration is the result of local excitation, 

 whether this be due to injury or to muscular contraction or both. 



LOCAL EFFECTS OF DIGESTIVE ACTIVITY ON SUSCEPTIBILITY. 



When recently taken food is present in a particular region of 

 the digestive tract a region of early disintegration appears cor- 

 responding at first to the region occupied by the food. Fig. 77 

 shows the case of a large individual of H. oligactis containing a 

 recently ingested Chironomid larva as indicated. The region of 

 the body in which the larva lies shows a very high susceptibility, 

 both ectoderm and entoderm disintegrating at the same time as 

 the tentacles and much earlier than other body regions. 



Figs. 78 and 79 show another somewhat similar case. This 

 animal, when isolated from the stock, had in its digestive tract 

 the shell of an entomostracan, the soft parts of which were wholly 

 or largely digested. An hour before the susceptibility deter- 

 mination entomostraca were placed in the dish with the animal 

 and one was ingested. In the figure the shell of the first ento- 

 mostracan is below the second newly ingested one. The body 

 region containing the second animal which is in process of di- 

 gestion shows a high susceptibility, both ectoderm and entoderm 

 disintegrating at the same time as the tentacles (Fig. 78), while 

 the lower region containing the animal which has already been 

 largely or wholly digested is less susceptible, but still more sus- 

 ceptible than other regions of the body and stalk (Fig. 79). 

 After the stage shown in Fig. 79 the disintegration did not extend 

 from the regions concerned in digestion, but progressed basipet- 

 ally from the hypostome and also began at the foot and progressed 



