FORM-REGULATION IN CERIANTHUS. 275 







always much longer than those of winter specimens even when 

 the latter were not atrophied. 



These facts leave no room for doubt that the reduction of the 

 tentacles and the loss of their tips is a normal phenomenon 

 during the winter months. I was able, however, to bring about 

 this change experimentally in September and October, i. e. t at a 

 time when newly taken specimens showed no trace of such 

 a change. 



Unfortunately it was impossible to continue observations 

 during the spring. I have no doubt, however, that as the tem- 

 perature of the water rose the internal pressure became greater 

 and the tentacles increased in length again. 



The animals do not thrive in aquaria with standing water and 

 it was not practicable to supply the aquaria with running water 

 kept at a constant temperature approaching that of the water in 

 summer. For these reasons the experiment of keeping the 

 animals with reduced tentacles in water of a higher temperature 

 than that in which they had been living could not be carried out 

 satisfactorily. I venture to predict, however, that the result of 

 such an experiment would be an elongation of the reduced ten- 

 tacles, though probably they would never attain the full summer 

 length unless the animals were well fed. 



There is no ground for supposing that the specimens with 

 atrophied tentacles were in a pathological condition. Specimens 

 which were in this condition when taken lived for months in 

 captivity and regenerated as completely as other specimens. 

 After atrophy of a certain portion of a tentacle the atrophied por- 

 tion was gradually absorbed and the remaining proximal portion 

 retained a perfectly healthy appearance until the gradual decrease 

 in internal pressure due to exhaustion brought about a further 

 atrophy. These reduced tentacles could be distinguished at 

 once from normal tentacles by their blunt tips. 



The only conclusion possible in view of the facts appears to 

 me to be that any condition which reduces the internal water- 

 pressure to a sufficient extent will bring about the atrophy of the 

 tentacles. Experimentally reduction of pressure was accom- 

 plished by preventing closure, but it is clear that any condition 

 which decreases ciliary activity will decrease the internal pres- 



