i^' SYMBIOSIS 299 



In Hydra viridis the endoderm-cells (Fig. 74, d) contain 

 chromatophores ^ (chr) coloured green by chlorophyll, which 

 performs the same function as in plants (p. 247), so that 

 in this species holozoic is supplemented by holophytic 

 nutrition. In H. fusca bodies resembling these chrom^to- 

 phores are present, but are of an orange or brown colour, 

 and devoid of chlorophyll. 



Muscle-processes exist in connection with the endoderm 

 cells, and they are said to take a transverse or circular 

 direction, i.e., at right angles to the similar processes of 

 the ectoderm cells. 



A\'hen a water-flea or other minute organism is swallowed 

 by a Hydra, it undergoes a gradual process of disintegration. 

 The process is begun by a solution of the soft parts due to 

 the action of a digestive fluid secreted by the gland- cells of 

 the endoderm ; it is apparently completed by the endoderm 



' In the substance of certain animals — e.g. , the order Kadiolaria of 

 the class Rhizopoda (p. 286), small Ilosmatococcus-like cells occur, 

 \\hich have been proved to be independent organisms, called Zoo.ran- 

 thclla. Such a living together of two organisms is known as symbiosis. 

 It differs essentially from ])arasitism (see p. 271), in «hich one organism 

 ])reys upon another, the host deriving no benefit but only harm 

 from the ])resence of the parasite. In symbiosis, on the contrary, the 

 two organisms are in a condition of mutually beneficial partnership. 

 The carbon dioxide and nitrogenous waste given off by the Radiolarian 

 serve as a constant food-supply to the Zooxanthella : at the same time 

 the latter by decomposing the carlion dioxide provides the Radiolarian 

 with a constant supi)ly of oxygen, and also with two important food 

 stuffs — starch and [jroteids, which, after solution, diffuse from the proto- 

 plasm of the Zooxanthella into that of the Radiolarian. The Radio- 

 larian may thercfcnc be said to keep the Zooxanthellie constantly 

 manured, while the Zoo.xanthella; in return supply the Radiolarian 

 \\ ith abundance of oxygen and of ready-digested food. There is some 

 reason for believing that the chromatophores of Hydra viridis are also 

 to be regarded as symbiotic organisms. 



