598 SHEINA MARSHALL 



are usually dropped after a few seconds, although a few 

 Hydras were induced to swallow pieces of white of egg (which 

 were returned undigested in the course of twelve hours) and 

 pieces of blotting-paper when soaked in blood. 



IfDaphniais left in a weak solution of litmus for eighteen 

 to twenty-four hours, the alimentary canal in the head-region 

 usually takes on a pink tinge. If these stained animals are fed 

 to Hydra, a colour-change from pink to blue takes place 

 in those animals which show nematocysts sticking into the 

 head-region. This may indicate an alkaline reaction for the 

 nematocyst poison. The change takes place slowly, often after 

 the death of the Daphnia, but does not occur when the 

 animal is killed with a needle. Simocephalus does not 

 take up the colour well, and often dies if left in the litmus 

 solution for twenty-four hours. 



The digestive juices of Hydra are alkaline, but have 

 no effect on Hydra itself. I have seen one Hydra com- 

 pletely ingested by another, in whose cavity it remained for 

 more than twelve hours. It was returned again none the 

 worse. A tentacle is frequently swallowed along with the 

 food to which it is sticking, and remains in the coelenteron 

 for some hours, but comes out quite unaltered. One Hydra 

 even went as far as to engulf about half its own body, beginning 

 at the foot, where a Daphnia had stuck. 



In many cases, particularly where the animal was attempting 

 to swallow something exceptionally large, the hypostome w^as 

 turned inside out over the tentacle-bases and remained so for 

 some time. It rarely went further than this, but in one small 

 regenerating specimen the process went on till the whole 

 animal had turned inside out. It righted itself in the course 

 of an hour. The converse also takes place sometimes, and the 

 hypostome is turned inwards till it hangs down into the coelen- 

 teron. Both of these performances are of interest, inasmuch 

 as a condition is assumed which has become permanent and 

 normal in other types of Coelenterata, e.g. the trumpet- 

 shaped hypostome of belia and the invaginated hypostome 

 of the Actinozoa. 



