VI.] THE FRESH-WATER POLYPES. 345 



coiled or looped filament which can be unrolled, present- 

 ing the appearance of a long filament attached to the 

 capsule, and often provided with recurved spines near its 

 base. As similar capsules of a larger size are the agents 

 by which many of the jelly fishes 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 

 its prey. Very rarely, nematocysts are to be found in 

 individual cells of the endoderm; there is reason to believe 

 that they are introduced with the captured prey, but argu- 

 ment from analogy to allied hydroids renders it probable 

 that they may be developed in situ. 



The chlorophyll granules contained in the endoderm of 

 the green Hydra are doubtless functional in the manner of 

 those of the plant-cell, but none but faint traces of an 

 'assimilation product' have yet been observed. The brown 

 or orange-coloured particles predominant in the endoderm 

 of the other species, and rarely present in that of H. 

 viridis, are probably identical with the chlorophyll bodies 

 (see Laboratory work) \ 



The larger endoderm cells of Hydra are throughout life 

 amoeboid, and the like is partly true of the ectoderm in at 

 least the young state of one variety {H. viridis var. Bakeri). 

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

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

 sac and have undergone a certain amount of metamor- 

 phosis. 



The cavity of the body alone represents a stomach and 



1 It has been assumed, upon this, that the green and brown species 

 are mere varieties of one and the same. On the other hand, structural 

 differences in the nematocysts and their parent-cells have been claimed, 

 as sufficient to justify a sulxlivision into three species. 



