Marvels of the Universe 



265 



wa\"ing about in the water. If carefully watched, they ma}- be observed to move about, 

 which they do either by bending over and taking hold of the surface with their tentacles, then 



letting ,go with the other 



end, and attaching it again 



close b}^ their new holci, or 



bv gliding over the surface 



on their end of attachment, 



much in the same way as a 



snail crawls Care must be 



taken not to jar the vessel or 



to disturb tne water, or the 



Hvdra will draw in its ten- 

 tacles and contract its body 



to a mere globule of dark 



green jelly and fall to the 



bottom. 



PluXotyl [-1. Uiil. 



Hydra swallowing a water grub. 



[J. Lenl. 

 allowing a young fisK. 



In spite of its e.xtreme tenuity, the whole animal is hollow, the cavity occupying the thread-like 

 body sending a branch up each of the tentacles ; its only opening is the mouth, which is situated 

 in the centre of the circlet of arms. Further, the body-wall of Hj'dra is composed of three 

 distinct layers ; the middle one of these is of a jelly-like consistency, the layers on either side of 

 this, both \\'ithin and without, being composed of cells. The cells of the inner layer project into the 

 central cavity, which serves as the stomach of the animal, and their secretions act upon the small 

 animals captured and swallowed by the Hydra, so that they can be digested. The cells of the outer 

 layer are considerably smaller, and are mainly muscular in function, effecting the elongation and 

 contraction of the aiimal, though among them are various kinds of special cells, such as the 

 reproductive cells and tha stinging cells. The latter are arranged in little groups, or batteries, 

 mainly upon the tentacles, and upon receiving the requisite stimulus, such as is afforded by the 

 touch of any passing creature, they shoot out a fine thread-like dart that pierces the body of the 

 prey and appears to exercise a paralysing effect upon it, so that it can be seized by the tentacles 

 and swallowed without further struggling. These darts, highly magnified, may be seen in the 

 photograph on page 264. \\'ater-ileas and other minute animals form the staple food of the Hydra. 

 A Hydra wiU very' often be observed to have one or perhaps two young ones growing out like buds 

 from the main body, at about one-third of its length from the base. The buds remain attached 



t) the parent for seme tiir.e, 



and put! out their tentacles 



and catch food on their own 



account, though their central 



cavity still remains in com- 

 munication with that of the 



parent. They may even put 



out secondary buds of their 



ov/n, so that we find one 



individual consisting of three 



generations, all enjoying a 



common stomach which 



branches up between them. 



The Brown Hydra found on 



the undersides of water-lily leaves and such-like places is more prolific in this manner than the 



Green Hydra of the duckweed ; it is also rather larger and more slend?r, and a well-budded 



I'holnln/l [.-1, U,il, 



One Hydra swallowing another. 



I'hnlu 4 ,] 



Hydra captui 



[J. Lcid. 

 a water-flea. 



