28 INTRODUCTION TO ZOOLOGY CHAP. 



into the digestive cavity a fluid which partially dissolves and 

 digests the soft parts of the food, which is then absorbed in 

 the liquid state by the adjacent cells. 



Then also the minute solid particles of food may be 

 directly engulfed by the blunt processes of the cells, just as 

 in Amoeba, and in this case the digestion takes place within 

 the cells. The skin cells are nourished by the liquid digested 

 food, which soaks through from one layer of cells to another. 



In the green and brown Hydrae, the digestive cells con- 

 tain the special pigment corpuscles which colour the body. 



In the case of H. viridis the green colouring 



matter is chlorophyll, the same pigment that is 



found in plants ; and by virtue of its presence, the Hydra, 



though an animal, can to some extent feed holophytically (see 



p. 21). 



Interesting investigations have been made into the nature 

 of these green corpuscles, and it has been shown that they 

 are really independent, unicellular plant -organisms of the 

 genus Zoochlorella, which are living inside the body of the 

 Hydra. Their presence is to the mutual advantage of both 

 plant and animal, for whilst the plants aid in the nutrition 

 of the animal, they gain in return a safe shelter, where they 

 have no lack of water containing carbonic acid and dissolved 

 food matter. Such a union of distinct organisms for their 

 mutual benefit is known as symbiosis. 1 



Undigested matter is driven out of the body 

 through the mouth, in the current kept up in the 

 liquid that fills the body-cavity, by the lashing of the whip- 

 like processes of some of the digestive cells. 

 Reproduc- Sometimes, especially when food is plentiful and 



tion by growth rapid, a Hydra multiplies by the very 

 Budding. s i m pi e method of budding. A small lump appears 

 at some point in the side of the body, and this grows out- 

 wards. When examined, it is found to be formed of a lateral 

 extension of the body-cavity, which pushes the double body- 

 wall in front of it, the cells rapidly dividing to keep pace 

 with the growth (Fig. 11, A, b). The outgrowth narrows at 

 its free end, a mouth appears, and round this a ring of 

 hollow tentacles develops, so that in time the bud takes on 



1 Zoochlorella enters into a similar partnership with green, fresh-water 

 sponges. See Gamble and Keeble, Q.J.M.S. xlviii., 1904, p. 363. 



