x.] THE FRESH-WATER POLYPES. 101 



a larger size are the agents by which many of the jeily nshes 

 sting severely, just as nettles do when they are handled, there 

 is every reason to believe that the thread-cells of the Hydra 

 exert a like noxious influence upon the small animals which 

 serve as their prey. 



Thus, Hydra is essentially a cellular organism like one 

 of the lower plants, but differs from them morphologically 

 in the fact that its cells are not enclosed within cellulose 

 walls ; and physiologically, in the dependence of these cells 

 for their nutrition upon ready formed protein matter. The 

 function of the chlorophyll granules contained in the endo- 

 derm of the green Hydra, and of the brown or orange- 

 coloured particles in the endoderm of the other species, is 

 wholly unknown. 



The Hydra, again, may be compared to an aggregate of 

 Amceba, which are arranged in the form of a double-walled 

 sac and have undergone a certain amount of metamorphosis. 



It is possible that the longitudinal fibres connected with 

 the cells of the ectoderm may be specially contractile, and 

 represent muscles; but, however this may be, each cell has 

 its own independent contractility. No trace of a special 

 nervous system has yet been discovered, and the manner in 

 which the actions of the different parts of the Hydra are 

 combined to a common end, as in locomotion and the seizing 

 of prey, is not understood. 



The Hydra has none of the special apparatuses which are 

 termed sense-organs, or glands. The cavity of the body 

 alone represents a stomach and intestine; there are no 

 organs of circulation, respiration or urinary secretion; the 

 products of digestion being doubtless transmitted, by im- 

 bibition, from cell to cell, and those of the waste of the cells 

 exuded directly into the surrounding water. 



