g5 HYDRA. PHYLUM CCELENTERATA 



difficulty in experimental work with Hydra is that one can never 

 be quite certain whether the animal is responding in a particular 

 way to the stimulus one has just given it, or merely doing so 

 because of an internal situation of which one knows nothing. 

 Besides the contraction already described, and the locomotion 

 and feeding mentioned below, there is another reaction in which 

 a tentacle kinks suddenly through a right angle or more, and then 

 slowly straightens. 



ENDODERM 



In the endoderm the cells are tall and columnar. Some of them, 

 especially numerous in the oral cone and absent from the tentacles, 

 are glandular. They have a narrow stem and a wide end, turned 

 towards the enteron and containing granules of a substance 

 which they secrete. The most numerous and conspicuous cells are 

 nutritive. They are stout, 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 con- 

 sists of a central mass of protoplasm with a covering containing 

 the green substance known as chlorophyll, to which the colour of 

 plants is due. These bodies multiply by division. In the other 

 species there are no green bodies, but there are present some 

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

 can be made out. The ends of the cells which abut on the enteron 

 bear fiagella, which can be replaced by pseudopodia. There are 

 some sense cells and a few isolated nerve cells. 



The green bodies, known as zoochlorellae, are non-flagellate 

 individuals of a minute plant related to Chlamydomonas, of the 

 genus Carteria. Like other green plants they nourish themselves 

 by building up complex organic compounds from simple inorganic 

 ones (carbon dioxide, water, salts, etc., see p. 20). They pre- 

 sumably obtain these simple substances as waste products of 

 the metabolism of the hydra. It may be that the hydra absorbs 

 from them in return the excess of carbohydrates which they 

 form ; and this would account for the absence from them of 

 starch, which is so constantly found in plants. Thus there would 

 be between the two. organisms a partnership, in which the animal 

 benefited by the removal of waste products and the supply of 



