No. 569] 



I{ E GENE It A TION 



285 



omy occurred in at least sixty-five per cent, of Thyone, 

 under the conditions described. Probably one reason why 

 this process did not occur in a still larger number is that 

 some animals occupied more favorable positions in the 

 aquarium. A discussion of the cause of evisceration will 

 be given later. 



When evisceration occurs it is sometimes hard to see 

 just how the process takes place. Pearse ('09) ascribes 

 the process to a " structural accident"; that is, it is due 

 to a powerful contraction of the circular muscles at a time 

 when the calcareous ring is well forward. ''But if the 

 tentacles are extended," he says, "and the calcareous 

 ring is pushed forward a break may occur at b" (a point 

 in his Fig. 2 where the body wall joins the calcareous 

 ring) "as a result of the strong contraction of the circu- 

 lar muscles at that point, and the visceral organs are 

 forced out. . . . Whether this autotomy takes place or 

 not depends upon the breaking of the inner branch of the 

 longitudinal muscle bands, whose normal function is to 

 retract the calcareous ring. When the strain brought 

 about by the contraction of the circular muscles becomes 

 too great these inner bands are torn asunder, usually at 

 the point x" (inner end of the retractors of the calcareous 

 ring). While it is true that muscular contraction and 

 consequent pressure undoubtedly plays a prominent part 

 in the process, close observation has convinced me that 

 this is not the only factor causing evisceration. Upon 

 several occasions I have watched carefully the breaking 

 of the body wall near its attachment to the calcareous 

 ring, and while there are times when the pressure appears 

 to be strong, especially when the animal is being irritated 

 mechanically, there are other times when the skin appears 

 to "melt away" or separate with very little or no pres- 

 sure present. Indeed, after the skin once breaks at one 

 side and the viscera escape through the opening, the pres- 

 sure is relieved. But one may observe that the skin con- 

 tinues to break until the calcareous ring is entirely sepa- 

 rated. This, of course, would not happen if the process 



