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. When the nematocyst is ejected these spines or barbs 

 lie in close apposition, with their pointed ends directed 

 forwards and their bases under tension ; on reaching the 

 attacked prey they undergo a forcible displacement, fixing 

 the body of the nematocyst and clearing the way for the in- 

 troduction of the more delicate thread. These thread-cells 

 appear to exert a noxious influence upon the animal's 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 argument 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) l . 



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. Baker /). 

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

 Amoeba, which are arranged in the form of a double walled 

 sac and have undergone an amount of metamorphosis. 



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 subdivision into three species. 



