Changes in the Life-cycle 317 



tadpole is prevented by keeping the animals in cold water, the 

 hind legs may continue to grow larger and their bones become 

 harder. The long spiral intestine shortens, as it does when the 

 tadpole changes into the frog, but the sexual organs do not 

 mature. 



How extensive the phenomenon of neotenia occurs in other 

 amphibia is not known. It has been suggested that forms like 

 Necturus, that always have external gills, do not rep resent more 

 primitive members of the group, but are neotenic species that 

 never undergo a metamorphosis, and become sexually mature 

 in the larval stage. In other groups of the animal kingdom the 

 same phenomenon occurs, according to some zoologists. Thus 

 it has been suggested that the rotifers represent the sexually 

 mature trochophore larval stage of the annelids, which has lost 

 its power to transform. Similarly, Appendicularia has been 

 supposed to be a sexually mature larval form of an ascidian, 

 and it has been suspected that even Amphioxus may be a neo- 

 tenic form. 



Boas has brought together many cases in which special or- 

 gans remain in a lower stage of development, while other organs 

 in the same animal undergo the characteristic transformation. 

 Thus we seem to have three forms of neotenia : one in which the 

 body remains at a lower stage and the sexual organs ripen; 

 another in which some of the organs advance (as in the tadpole), 

 but sexual maturity does not supervene; and third, those in 

 which some organs fail to develop at the time of sexual maturity. 



A case that appears to be a parallel one in some aspects exists 

 in the white ants according to Grassi. The members of the 

 colony consist of a royal couple, workers, soldiers, and young. 

 If one of the royal pair is removed, a royal substitute form is 

 reared, either male or female, according to which is needed. 

 If both royal individuals are removed, two royal substitutes are 

 reared. Grassi has shown experimentally that this occurs, but 

 the way in which the transformation is effected was not deter- 

 mined, although he thinks that the amount or the kind of food 

 given to the young determines the result. The young appear 



