COELENTEEATA 35 



the brown Hydra is said to be due to small particles of an 

 excretory nature accumulated at the base of the endoderm. 

 In the green Hydra this region of the endoderm is occupied 

 by green cells of an alga called Zoochlorella, a green alga 

 which in this case, as in some Protozoa and other lowly 

 Metazoa, is able to resist digestion and to live symbiotically 

 within the animal cell, as the allied Xanthella does in so many 

 marine Coelenterates and Protozoa. In the endoderm of 

 Hydra the algal cells obtain protection and the food they 

 require. Hydra is relieved of excretory material, and the 

 carbon dioxide liberated by the animal is split up by the 

 plant, the carbon utilised by the alga, and the oxygen by 

 Hydra. 



Many cells of the endoderm of the hypostome are glandular, 

 and these contribute during the ingestion of food to maintain- 

 ing a digestive activity of the watery fluid of the enteric 

 cavity, and the ferments of the fluid of the gastric cavity 

 release the food from the hard cases of small Crustacea and the 

 like, which are ejected in the entire state but robbed of the 

 digestible contents. When the food is introduced into the 

 gastric cavity it is disturbed by the lashing movements of the 

 flagella. This action brings the food into the requisite position, 

 and then the flagella are withdrawn and pseudopodial extru- 

 sions are produced for the purpose of bringing the food within 

 the endoderm cells. Digestion is completed, therefore, intra- 

 cellularly. Food vacuoles are formed, and the digestion 

 proceeds as in a Protozoon. When the food is reduced by 

 fermentative action to a fluid state it is distributed by osmotic 

 currents to neighbouring cells, and it must evidently pass 

 through the mesogloea to gain the cells of the ectoderm. The 

 undigested remains of the food are egested through the mouth. 

 The food consists of small Crustacea and young Crustacea, 

 insect larvae, and worms. 



The general physiological processes of Hydra are very 

 similar to and but little advanced beyond those of the Proto- 

 zoon. The differentiation into a protective, sensory ectoderm 

 and a digestive layer shows a division of labour, expressed by 

 morphological change. It is evident also that the ectoderm 

 is supplied with food by the endoderm ; there is no circulatory 



