Behaviour of Hydroids 43 



immediately starts a bout of swimming. 1 He 

 states that when the margin of the bell, together 

 with all its ocelli are removed, the swimming 

 bell, although still able to contract, no longer 

 responds to luminous stimulation of any kind or 

 degree. But if some of the ocelli are left in 

 situ, light induces an unfailing response of the 

 entire animal. 



We may now pass on to consider the move- 

 ments of Hydra in order to determine how far 

 they are directed by instinctive processes. 

 Although a Hydra is usually attached by its 

 foot to some solid substance, its body and 

 tentacles undergo constant contractions and 

 expansions; these movements are augmented 

 by any vibrations, such as those caused by 

 slamming the door of the room in which the 

 aquarium containing the Hydra is placed. 2 

 Reaction to stimuli of this kind leads to move- 

 ments which enable the Hydra's tentacles to 

 explore the surrounding water, and thus increase 

 its chance of capturing any suitable object which 



1 Jelly-Fish, Star-Fish, and Sea-Urchins, by G. J. Romanes, 

 M.A., LL.D., F.R.S., p. 39. 



2 The Quarterly Journal of Microscopic Science, New Series, 

 No. 192, p. 585, G. Wagner on "Some of the Movements and 

 Reaction of Hydra." 



