98 



HYDRA. PHYLUM CCELENTERATA 



with the naked eye. The brown hydra, when starved is able to 

 feed on nmd. The animal will not feed unless it be hungry. If 

 it be well fed, creatures which swim against the tentacles are 

 allowed to escape, but. if food has been scarce, as soon as the prey 

 has become temporarily attached by the nematocysts to one ten- 

 tacle the others bend over towards it and help to secure it and 

 push it towards the mouth. If the animal be starvmg the mere 

 smell of food in the neighbourhood is enough to set the tentacles 

 working, but usually they are not put into action till the food 

 has been both smelt and touched. It is not possible to deceive 

 the hydra into swallowing substances, such as pieces of blottmg- 



YiG. 64. — Hydra in the attitudes which it assumes successively in 

 two of its modes of locomotion. 



A , ' Looping ' ; B,' somersaulting'. 



paper, which do not smell like food, but blotting-paper soaked 

 in beef-tea is swallowed when it touches the tentacles. Once 

 swallowed, the food is passed deep into the enteron and there 

 softened by a juice which the gland cells secrete, broken up by 

 the churning which it gets as the body expands and contracts, 

 and swept about by the fiagella. Part of the food is dissolved 

 in the enteron and absorbed in solution, part of it taken up by 

 pseudopodia of the endoderm cells and digested within their 

 protoplasm. Presumably the ectoderm is nourished by substances 

 passed on from the endoderm, either by diffusion through the 

 structureless lamella or along the fine threads of protoplasm 

 which put the two layers into connection across it. The undigested 

 remains of the food are driven out of the mouth by a sudden 

 contraction of the wall of the body. In unnatural conditions of 

 culture the animals become liable to depression much like that 



