98 ELEMENTAKY BIOLOGY. [X, 



exert a like noxious influence upon tlie 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 inclosed 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 endoderm, 

 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 

 Amcebce, 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 re- 

 present 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. 



