HYDRA AND OBELI A. POLYPS AND MEDUSA 167 



substance which they secrete. The most numerous and 

 conspicuous cells are nutritive. They are columnar, and 

 have their bases produced into contractile fibres, which are 

 shorter than those of the musculo-epithelial cells and run 

 around the body, not along it. Their protoplasm contains 

 large vacuoles, and also, in the green Hydra, a number of 

 round bodies of a green colour, each of which consists of a 

 central mass of protoplasm with a covering of a different 

 kind of protoplasm containing the green substance known 

 as chlorophyll to which the colour of plants is due. These 

 bodies multiply by division. In the brown Hydra the green 

 bodies are absent, but there are present some yellowish 

 bodies of similar shape, in which, however, no structure can 

 be made out. It is said that these are also present in the 

 green Hydra. The ends of the cells which abut on the 

 enteron bear flagella, which can be withdrawn and replaced 

 by pseudopodia. 



The green bodies of Hydra viridis have been shown to 

 be individuals of a minute plant known as Zoochlorella, 

 which have simple and degenerate structure. Like other 

 plants, they nourish themselves in a manner radically 

 different from that of animals. This they do by means 

 of the chlorophyll. Absorbing certain rays of light, that 

 substance enables the protoplasm, in some way not yet 

 understood, to use the energy of such rays in breaking up 

 molecules of carbon dioxide taken in from the surroundings, 

 which in the case of land plants is the air, in water plants is 

 the water, and in the green bodies of Hydra is the proto- 

 plasm of the animal. The carbon thus obtained is combined 

 with the hydrogen and oxygen of water also absorbed, to 

 form sugar. 1 The oxygen of the carbon dioxide is set free. 

 This can easily be shown in the case of water plants, from 

 whose leaves in sunlight a stream of fine bubbles of oxygen 

 may be seen to ascend. It has been shown that oxygen is 

 also given off from the body of the green Hydra. The 

 sugar is used on the one hand for the manufacture of the 

 carbohydrates, in which the plant body is usually very rich, 

 and on the other hand for the formation of the various 

 substances which the protoplasm of plants, like that of 

 animals, requires for food, and in particular of proteins. 

 1 The substance first formed is Formaldehyde, H.COH. 



