HYDRA: ITS ANATOMY AND DEVELOPMENT. 245 
flattened disc, which, by its suctorial power, enables the animal to 
attach itself to various bodies. No canal can be found in the disc, 
but it is slightly hollow, and has a dark appearance when in the 
flattened state. 
There are three well known species. Aydra vulgaris. Body 
orange brown, yellowish, or red in colour ; cylindrical in shape, 
having from seven to twelve tentacles as long or longer than the 
body, which, even when not extended, tapers to the free ends. 
This species is common. 
Lfydra viridis. Body, leaf green, cylindrical, or gradually 
narrowing towards the base. Has from six to ten tentacles, shorter 
than the body, and narrowest at their origin. ‘This species is also 
common. 
Hydra fusca. Body, brown, or greyish. Lower half suddenly 
attenuated. Tentacles five to eight in number, several times longer 
than the body. Rare in some parts. 
Another, and very rare species, is the Hydra attenuata. Body 
pale olive green. Posterior part attenuated. Tentacles pale, and 
considerably longer than the body. 
The green colour of & viridis, and HZ. attenuata, undoubtedly 
results from the presence of chlorophyll grains, embedded in the 
protoplasm of the middle cells, although some writers express a 
belief that wherever chlorophyll is really found in the animal 
kingdom we have to do with green algze living in the animals. 
By various experiments I have found that the colour of all the 
species is much affected by light. ‘Those which are exposed to 
direct sunlight become bleached. But upon placing them out of 
the rays of the sun the colour returns. I find that the best way to 
keep Hydra alive is to darken the bottom of the tank with card- 
board about half-way up, so that if the Hydra prefers it can retire 
in the shade, and this proceeding also prevents the animal from 
burying itself in the mud. 
Dividing Hydra viridis makes it lose much of its colour. ‘The 
colour of all Hydra is also modified by the nature of its food. 
Laurent states that he succeeded in colouring them blue, white, and 
red, by feeding them with indigo, chalk, and carmine. I have, by 
colouring the water with indigo, carmine, etc., coloured the animal, 
and found it not only lived, but thrived well enough to bud. It 
will not, however, retain the artificial pigment, but will almost at 
once return to its original colour when removed from its influence. 
( Zo be continued. ) 
