122 LECTURE VII. 



mass of excrementitious matter composed of a tenacious mucus im- 

 bedding a granular substance resembling both in colour and texture 

 that which lined the digestive cavity. Such pore may give passage 

 to certain excretions of the lining membrane of the cavity, but the 

 coarser indigestible parts of the prey are habitually regurgitated by 

 the mouth. 



Each tentaculum in the Hydra grisea, according to Trembley, is 

 a tube, which communicates with the common digestive cavity. The 

 food which is rotated in that cavity is driven up some way into the 

 tentacles, and sent back again.* Cordaf found the walls of the tenta- 

 cular cavities to contain a fluid albuminous substance mixed with oil- 

 like particles. This substance swells out at certain definite places 

 into denser nodules, which are arranged in a spiral line (Jig. 60, a, a). 

 Each nodule is furnished with tactile filaments, and a singularly con- 

 structed organ for catching and wounding the prey. The parts re- 

 garded by Corda as organs of touch consist of a fine sac, inclosing 

 another with thicker parietes, and within this there is a small cavity : 

 from the point where the two sacs coalesce above, there projects a 

 long and very slender filament, which is non-retractile. The wounding 

 or seizing organ consists of an obovate transparent sac, immersed in 

 the nodule with a small aperture. At the bottom of the sac, and 

 within it, there is a solid corpuscle, which gives origin to a clear 

 calcareous sharp dart or spine, that can be pushed out at pleasure, or 

 withdrawn until its point is brought within the sac. When the 

 hydra wishes to seize an animal, the darts are protruded, by which 

 means the surface of the tentacula is roughened, and the prey more 

 easily retained: Corda believes that a poison is at the same time 

 ejected. The nodules of the tentacula are connected together by 

 means of four muscular bands, which run up, forming lozenge-shaped 

 spaces by their intersections : these are joined together by transverse 

 bands. J The lip of the mouth is armed with darts similar to those 

 of the tentacula ; and they have been found in the skin of the body 

 and of the foot. 



That the tentacula have the power of communicating some be- 

 numbing or noxious influence to the living animals which constitute 

 the food of the Hydra, is evident from the enect produced, for 

 example, upon an entomostracan, which may have been touched, but 

 not seized, by one of these organs. The little active crustacean is 

 arrested in the midst of its rapid, darting motion, and sinks, appa- 

 rently lifeless, for some distance ; then slowly recovers itself, and 



♦ CI. torn. i. p. 260., tomii. p. 228. t XCIX. 



X CIV. p. 35. 



