INVERTEBRATA. 



381 



an ectoderm-cell. The free surface, abutting on the enteron, 

 is amoeboid, projecting either into blunt pseudopodia or 

 into long thread-like ones called flagella. A flagellum is 

 intermediate in character between a pseudopodium and a 

 cilium. 



The pseudopodia can, like those of Amoeba, ingest 

 small particles of food, so that part of the process of diges- 

 tion takes place within the protoplasm of the endoderm cells 

 (ultra-cellular digestion) ; but not all. For the endoderm 

 cells of the hypostome, which are smaller than the rest, are 

 actively secretory. There can be little doubt that they 

 secrete a juice something like the rabbit's pancreatic juice, 

 containing one or more ferments, and that part of the 

 digestion is carried on by this secretion in the enteron 

 (enteric digestion). 



The food of Hydra is not limited to organisms large 

 enough to be grasped by the tentacles. Diatoms may also 

 be found ingested in its endoderm cells : in the absence of 

 true cilia, we must suppose these to have been brought in 

 by a current of water caused by the flagella. 



8. Chlorophyll. There are three generally recognized 

 common species of Hydra, distinguished principally by the 

 difference of colour of the endoderm, which shows through 

 the transparent ectoderm, and gives an apparent colour to 

 the whole animal. Thus Hydra viridis is green ; H. fusca, 

 yellow-brown ; H. vulgaris, colourless. In the first of 

 these the green colouring-matter is none other than chloro- 

 phyll, so characteristic of the vegetable kingdom, and it 

 occurs in specialized portions of protoplasm, which are 

 identical in character with the chromatophores (chloro- 

 plastids) of plants.* 



There can be no doubt that the nutrition of Hydra 

 viridis is partly effected in the manner of a plant, viz. by 

 the utilization of sunlight-energy, in building up organic 

 compounds from simple inorganic ones (holophytic nutrition), 

 as well as by the purely animal method (holozoic nutrition). 

 In proof of this, we find starch present in the endoderm- 

 cells. Attempts have been made to show that these chroma- 



* See Text-Book of Botany, chapter ii., 9 and 14. . . 



