114 CHAS. B. WILSON. 



In another species the posterior fragment began to develop 

 a new head, and he figures several stages in the process, 

 which occupied nearly three months but was not completed. 



Barrels also has described the regeneration of headless 

 trunk-pieces in another species, and they have been known 

 to regenerate new heads in the common Linens socialis. 



Fission certainly produces very little apparent effect upon 

 vitality. I have repeatedly obtained headless fragments of 

 both sexes containing unripe sexual products and kept them 

 until they ripened, a period sometimes of two or three weeks, 

 and then fertilised the eggs from one fragment with the 

 sperm from another, and obtained perfectly healthy pilidiums 

 as a result. 



Of course the posterior fragments can eat nothing, but the 

 anterior ones display as voracious an appetite as ever imme- 

 diately after fission, or even during the process. This is attested 

 by the preserved specimen whose photograph is shown in 

 fig. 1. The prolonged retention of sexual functions by head- 

 less fragments under adverse circumstances gives some idea 

 of the vitality possessed by these worms in their native 

 haunts, but repeated search has failed to find a single one in 

 which regeneration has even commenced. 



These facts, taken in connection with those developed in 

 the aquarium, lead to two conclusions. First, that Cerebra- 

 tulus frequently dismembers at the close of the breeding 

 season ; and second, that while the anterior fragments regu- 

 larly regenerate, the posterior ones seldom if ever do so. 



Professor Benham, of Oxford, has noted a similar case of 

 spontaneous dismemberment in the genus Carinella (10). 



Here also there was a relation between fission and genital 

 maturity, but of a different nature from that in Cerebratulus. 



He found that the genital elements in Carinella were pre- 

 sent only in the posterior regions of the body, and that this 

 portion was constricted and cut off as fast as the elements 

 ripened. 



Fission in this genus, therefore, would be a method of 

 discharging the ripened sexual products. 



