AESTIVATION OF BOTRYLLOIDES GASCOI DELLA VALLE. 161 



still further resembled von Drasche's species. Here, then, it is evident that as a single 

 colony successively assumed the characters of two described species these two must be united 

 into a single one, B. gascoi Delia Valle.* 



4. Cause of Estivation — I think that during the entire period of aestivation and 

 rejuvenescence the temperature of the water was slowly rising, so that it was hotter 

 when the colony had recovered than when it was activating. But my records are 

 not full enough to make this opinion certain. It seemed also that other conditions 

 in the aquarium were nearly constant, but no attempt was made to measure them. 

 Accordingly, I cannot say whether the aestivation was a contrivance for tiding over 

 a rapid unfavorable change in the external conditions and rejuvenescence took place 

 upon the re-establishment of the former state of things, or whether the environment 

 was practically the same but unfavorable, and the aestivation and rejuvenescence 

 were processes which actually gave increased vigor to the colony and enabled it to 

 cope with surroundings which formerly it could not deal with. I think it probable, 

 however, that the latter alternative was the true one. 



5. Hibernation and Estivation. — As the process here described is so closely 

 akin to hibernation, it may be well to see in how far the general results that have 

 been arrived at concerning wintering apply to this case. Among the authors who 

 have studied hibernation Caullery ('95), more than any one else, has discussed the 

 general aspects of the question, and, on that account, I will consider chiefly his con- 

 clusions. He says ('95, p. 28) that we must renounce the conception of latent life 

 in the colony and the formation of special dormant buds as a response to the stimulus 

 of the cold. The changes which take place he thinks are due to other causes than 

 the cold. Even when they occur in winter (which is not always the case) they are 

 more probably brought about by a senescence of the colony after sexual reproduc- 

 tion; and then winter appearing retards the new development of the colony. The 

 species among which the most pronounced cases of hibernation occurred were mem- 

 bers of the Polyclinidae. Ordinarily that part of the colony containing the thoraxes 

 and abdomens of the zooids degenerated, leaving only the postabdomens, which then 

 formed the buds destined to produce the next zooids. In some species these buds 

 developed into adult zooids during the winter, in others they did not mature until 

 the next spring. 



In a later article Giard and Caullery ('96) describe the hibernation in Clavelina 

 lepadiformis Miiller. Here both zooids and buds entirely disappeared, and the colony 



* This variation is not surprising when compared with results at which I have arrived in the case of Botryllus. 

 It is probable that if the colony had been kept longer and put out in the bay it would gradually have regained its 

 former color. It may be that the red spots described by von Drasche in B. luteum were the first step in such a 

 color change. 



