338 ANIMAL BIOLOGY. [Part II. 



threads may be seen protruding here and there. On addition of 

 an irritant, such as dilute acetic acid, great numbers shoot forth. 

 They are of use in capturing (and perhaps paralysing) prey. 

 Some of these cells are shown in Fig. 100, F. The largest (1) have 

 spines at the base of the very long whip. Others have no 

 spines, and may be incompletely developed thread-cells. In the 

 resting stage (before eversion) the thread-cell is rounder, and 

 resembles a flask in which the tapering neck has been pushed 

 inwards so as to lie within the flask. Within the neck the 

 barbs appear as arrow-heads. The neck is continued into a 

 long filament regularly coiled like a ship's cable. It would 

 seem that during eversion the neck and the whole length of 

 filament is turned inside out. This process would, moreover, 

 seem to be a vital one, and within the command of the Hydra. 

 Infusorians (Euplotes) have been observed wandering over the 

 body and tentacles of Hydra fusca without causing any discharge 

 of nematocysts. The free end of a cell bearing a nematocyst is 

 generally prolonged into an elementary sensiferous process or 

 cnidocil. Connected with these cells are small stellate nerve-cells. 



Internal to the muscle processes and the delicate membrane 

 (supporting lamella) with which they enter into close relation, is 

 the endoderm layer, consisting, of large irregular cells (Fig. 100, 

 A. en., and D.). Their inner free surfaces carry cilia, while their 

 deeper parts are crowded, in Hydra viridis, with the green 

 chlorophyll corpuscles (M.\ which give to this hydra its dis- 

 tinctive colour. 



Concerning these chlorophyll corpuscles there has been much 

 discussion. By some they are regarded as plants (algae) para- 

 sitic (or rather symbiotic, i.e. living together for mutual advan- 

 tage) on the hydra. It is more probable, however, that they 

 are true products of the animal cells. One of these is shown at 

 E., as seen under the highest power, the parts shaded dark being 

 in the cell itself green. Very commonly the green part forms 

 (as seen in optical section) a ring surrounding a clear central 

 part of the cell. There is no nucleus. As in the case of 

 vegetable chlorophyll, the green colouring 'matter liberates 

 oxygen in sunlight, as has been proved by keeping hydras under 



