AUTONOMY IN HOLOTHURJANS. 43 



II. Mechanism of Autotomy. 



As Lukas ( : 05) has pointed out the place where self-muti- 

 lation takes place is usually determined definitely by the struc- 

 ture of an animal, and in the two genera under consideration the 

 process is quite different. Leptosynapta constricts off pieces at 

 the posterior end of the body (Fig. i) until there is often only a 

 small anterior fragment remaining. Such fragmentation is brought 

 about by the strong local contraction of the circular muscles 

 which pinch the body in two. 



Thyone never constricts off pieces at the posterior end. In 

 autotomy the body-wall breaks open just behind the calcareous 



Fig. I. Leptosynapta inhcerens, showing method of autotomy. 



ring, at the point indicated by b in Fig. 2, and the visceral organs 

 are thrown out. If the body is in the position shown in the 

 figure, however, there will be no autotomy for the contraction of 

 the circular muscles at c forces the calcareous ring {c.os}j back 

 into the body. But if the tentacles are extended and the calcareous 

 ring is pushed forward a break may occur at b, as a result of the 

 strong contraction of the circular muscles at that point, and 

 visceral organs are forced out. The calcareous, water-vascular 

 and nerve rings are thus ejected from the body, together with 

 the tentacles {i) and more or less of the alimentary canal {s, i). 

 Whether this autotomy takes place or not depends upon the 

 breaking of the inner branch {l.m.2) of the longitudinal muscle 

 bands {l.in.), whose normal function is to retract the calcareous 

 ring {cos.). When the strain brought about by the contraction 

 of the circular muscles {c.in.) becomes too great these inner bands 

 are torn asunder, usually at the point x. 



III. Dependence of Autotomy on the Presence of the 



Anterior Portion of the Body. 



In Leptosynapta (Fig. i) the manner of the fragmentation of 



the body points to an influence which perhaps comes from the 



circumoral nerve ring. Pieces are always constricted off from 



the body at the posterior end and fragments thus separated do 



