170 MANUAL OF ELEMENTARY ZOOLOGY 



animal be starving the mere smell of food in the neighbour- 

 hood 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 blotting- 

 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 endoderm secretes 

 (from the gland cells already mentioned) and broken up 

 by the churning which it gets as the body expands and 

 contracts. Part of the food is dissolved in the enteron 

 and absorbed in solution, part of it is 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. After a long period of high feeding 

 the animals become liable to depression much like that of 

 Paramecium, in which the powers of movement, feeding, and 

 fission are affected and death ensues. Respiration and ex- 

 cretion probably take place from the surface of the ectoderm 

 and endoderm ; there is no special organ for either process. 

 The species of Hydra reproduce themselves both 

 sexually and asexually. The sexual reproduc- 



Reproduotion. .. i tt ■ -j- j tt ». i i 



tion of H. viridis and H. grtsea takes place 



normally in the spring and summer, that of H. fusca in the 

 autumn. The animals are usually hermaphrodite, but strains 

 are met with in which the sexes are separate. The genera- 

 tive organs are ectodermal structures developed when 

 sexual reproduction is about to take place. The ovaries, 

 of which there is generally only one in each individual, are 

 found in the lower part of the body ; the testes, of which 

 there are several, are in the upper part. In the early 

 stages of both organs the interstitial cells multiply and push 

 out the musculo-epithelial cells so as to form a swelling. In 

 the case of the ovary one of the interstitial cells becomes an 

 oocyte (p. 1 06). This increases in size and begins to throw out 



