DIGESTION. 129 



especially Mobius, reject tbe idea of penetration, and maintain that the thread-cells arrest the 

 motions of the prey by the mere adhesion of the ejected filament. 



I believe, however, that it will be found that there is an actual jicnetration, but not to the 

 extent that is usually insisted on. If some soft body, such as a worm, be brought into contact 

 with the tentacles of a Hydra, the surface of contact may be seen immediately afterwards to be 

 covered with large discharged thread-cells, many of which will be found with the freed end of the 

 barbed sac inserted into it as far as the roots of the barbs. I have never, however, witnessed a 

 deeper penetration than this, and the barbs themselves were never immersed. 



I believe that in such cases the action of the thread-cell consists in the sudden ejection of 

 the barbed sac against the tissues of the prey, which, if these be soft enough, allow the point of 

 the sac to penetrate as far as the roots of the barbs. This act is instantly followed by the ejection 

 of the filament, for which the barbed sac has opened the commencement of a passage, and which 

 now worms its way among the tissues, recalling the mode in which the delicate filaments which 

 form the mi/cclium of certain parasitic fungi penetrate the organic structures infested by them. 



It is impossible, however, to believe that such effects as follow the action of the thread-cell 

 can be produced by mere mechanical penetration, and the conclusion is irresistible that the pene- 

 tration is accompanied by the injection of some potent virus which acts by rapidly destroying the 

 irritability of the living tissues, the tubular filament of the thread-cell as it continues to insinuate 

 itself, affording at the same time a channel by which the special secretion of the cell is conducted 

 into the deeper parts of the tissues of the prey. 



In judging of the functions of the thread-cells, however, it must not be forgotten that they 

 also occur in parts which preclude the possibility of their being employed as offensive or defensive 

 organs, as, for instance, when they exist, as they frequently do in great numbers, in that part of 

 the ectoderm which is under cover of the hard chitinous perisarc, or when they are included in 

 the interior of cavities which can have nothing to do with the capture of the prey, as in the 

 sac-like receptacles which occur in the umbrella of Gemmaria. 



Though we have scarcely yet sufficient data to enable us to determine the exact limits of 

 that part of the somatic cavity on which the function of digestion specially devolves, we shall 

 perhaps be justified in considering as such that portion of it which is included within the hydranth, 

 and which is almost always distinguished by its form and by some special structure of its walls 

 from that which belongs to the cmnosarc; while in the medusa the cavity of the manubrium, or 

 at least its basal portion, must also be regarded in a special sense as devoted to the digestive 

 functions of the free planoblast. 



When the food has once passed the mouth and entered the cavity of the hydranth, or that 

 of the manubrium of the medusa, it is there subjected to a process of solution. Of the nature 

 of the solvent we as yet know almost nothing. There can be .little doubt, however, that it is 

 secreted from the walls of the cavity; and though it must be more or less diluted with water which 

 has obtained admission through the mouth, its action on the food is powerful and rapid. This 

 process is doubtless aided by the motion to which the contents of the digestive cavity are subjected 

 by the contraction of the walls and the vibration of the cilia which clothe them. The soluble 

 and nutritious parts are speedily separated, and the insoluble and non-nutritious dchris are ejected 

 through the mouth.^ 



' Corda (" Anat. Hydra; fuscJE," in 'Ann. des Sci. Nat.,' 1H37) has described the body-cavity of 

 Hydra as communicating with the external medium by an orifice situated at the end opposite to tbe 



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