OBSERVATIONS ON HYDRA 597 



insect larvae, all of which they ate readily. Simocephalus 

 was also tried as a food, but the Hydra seemed unable to 

 kill it. The Simocephalus were frequently caught and held 

 struggling for an hour or more, but in the end they freed 

 themselves and escaped uninjured. If killed and presented 

 to the Hydra, they were eaten as readily as Daphnia. 

 On three occasions a Simocephalus was captured and 

 eaten by a Hydra. In the first case the animal remained alive 

 inside the Hydra for a considerable time and could be seen 

 moving its antennae. A second was caught and digested 

 immediately after ecdysis. I have, however, seen a Hydra 

 catch a Simocephalus which then underwent ecdysis 

 and was immediately recaptured, yet eventually freed itself. 

 When caught, the contrast between the behaviour of Daph- 

 nia and of S i m c e p h a 1 u s is remarkable. The Daphnia 

 struggles violently for a few minutes, then the heart stops 

 beating and the animal soon succumbs although the antennae 

 may keep up a quivering movement for some time longer. 

 If freed from the Hydra, it does not recover. Simo- 

 cephalus, on the other hand, continues to live and to 

 struggle at intervals until it frees itself. The heart continues 

 to beat the whole time. The cuticle of Simocephalus is 

 not appreciably thicker than that of Daphnia. It may be 

 more resistant to the entry of the nematocysts, or the tissues 

 of the animal to the action of their poison. When both 

 Daphnia and Simocephalus are immersed in dilute 

 solutions of poisons (such as formic acid, or chloroform) 

 Simocephalus succumbs first. 



Schulze (9) mentions that in a culture of H. circum- 

 cincta a Daphnia sometimes stuck on to the tentacles 

 and was dropped again uninjured. It is perhaps possible that 

 these were really Simocephalus also, for the two genera 

 are closely similar and may be mistaken for one another 

 unless carefully examined. 



The capture of the food generally seems to be a more or 

 less passive action, any small object presented to the Hydra 

 being seized and carried to the mouth. Indifferent substances 



