39 2 OSCAR RIDDLE. 



autotomy of healthy hydramhs more gradually in period- ex- 

 tended to one to four hours, and \ watch the course of the 

 process in a single individual during thi> time. From these 

 observations I may here record one or two point- \\hi.-h -eem to 

 throw some light on the reason for the histolysis just mentioned. 



I cite the case of a tubularian which \\.i- kept mechanically 

 stimulated by touching or pricking tin- hydrant h with a dissecting 

 needle and in which the process of autotomy had advanced at 

 such a rate as readily to separate at the end of four hours when 

 lifted from the sea-water. With the beginning of contraction 

 in this animal the circulatory current in the "neck" region was 

 stopped; indeed the circulatory-nutritive fluids were quite ex- 

 pelled and excluded from approximately two millimeters of this 

 region; the entodermic walls of the tube here being completely 

 and tightly apposed. The closure at this point also largely 

 stopped for a time the circulation through the stem. The fluids, 

 however, were as before in contact with the walls of the gastro- 

 vascular cavity everywhere except in the much contracted neck 

 region. That is to say, in the contracted animal the normal 

 nutritive fluids were in contact with all the structures with which 

 they are normally in contact except at one point the "neck" 

 region; it is always at this latter point that histolysis and autotomy 

 later occur. 



In a little less than an hour it was found that the dissepi- 

 ment which divides the gastro-vascular cavity of the stem into 

 two channels one for the anterior, the other for the posterior 

 flow of the circulatory fluids had been broken at a point a little 

 below the contracted "neck," and that the usual circulation of 

 fluids was again established within the stem. Soon however 

 there accumulated at the point immediately below the neck a 

 quantity of the red pigment and other debris from the circulation; 

 thus the channel became so firmly plugged that even a relaxa- 

 tion of the contracted neck region could not now effect a ree>tal>- 

 lishment of circulation in this "neck" region. 



It is my opinion, furthermore, that the reestablish ment of this 

 circulation after a few hours of contraction might be prexciiied 

 or at least greatly hindered by another circumstance if for 

 any cause the debris just mentioned should fail io collect and 



