VI.] THE FRESH-WATER POLYPES. 345 



of mutual inter-relationship between the plant and animal 

 known as symbiosis, in which each utilizes certain waste 

 products of the other, is probably maintained. There is 

 some reason for thinking that the brown or orange-coloured 

 particles predominant in the endoderm of the other species, 

 and less rarely present in H. viridis, may be perhaps 

 disintegrating Chlorelloids. 



The cells of the ectoderm, and especially of that of 

 the tentacles, contain very singular bodies, — the so-called 

 urticating capsules, thread cells, or nematocysts. These are 

 oval bags, with thick and elastic walls, containing a spirally 

 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 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 

 Amasbce, 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 



