44 



A. S. PEARSE. 



not again break up, as is the case in the arms of some ophuroids. 

 Furthermore, if an individual is cut in two in the middle of the 

 body the posterior half does not constrict off pieces, though it 

 undergoes irregular contractions and constricted rings are often 

 formed. The anterior half however will continue to fragment at 

 its posterior end. The results are the same if an animal is cut in 

 two in such a way that the posterior piece contains two thirds or 

 three fourths of the body. 



cm 7.m2 s c.os b 



U I I I / 



^■^- l.m. i c.m. X p. v. I,m.2 I.m.i 



Fig. 2. Thyone briareus. The right side of the body-wall as well as S'Tiie of the 

 dorsal portion have been removed. The ampullse and mesenteries are not shown. 

 b, point where body wall is ruptured in autotomy ; r, region where circular muscles 

 are effective in preventing autotomy ; cc, cloac^l chamber ; c.t?t.^ circular muscles ; 

 CO., cloacal opening; ^-.oj., calcareous ring; ?', intestine ; in., integument; l.m., 

 longitudinal muscle bands ; /w./, outer branch of longitudinal muscle bands ; 1. 711. 2, 

 inner branch of longitudinal muscle bands ; 7-.w., radial muscles which dilate the 

 cloacal chamber ; r.A, respiratory tree ; s, stomach ; t, tentacles ; x, weak point in 

 inner branch of longitudinal muscle band. 



Thyone s mutilating reflexes always involve the loss of the 

 nerve ring and the remaining portions of the body do not under- 

 go further fragmentation but ultimately regenerate the lost parts. 

 As in Leptosynapta there is no autotomy when the portion of the 

 body which contains the nerve ring is absent. 



IV. Effect of Chemical Substances on Autotomy. 

 In order to ascertain whether substances which increased the 

 general excitability would induce a corresponding increase in the 

 number of individuals manifesting autotomy, certain drugs and 



