HYDROIDS. 



75 



number, shape, and position in different species ; the remainder of the cavity of the 

 capsule is filled with a liquid very similar to, if not identical with, formic acid. Now, 

 when any stimulus brings a cnidocell into 

 activity, it forcibly ejects the larger part 

 of the tube by a process of evagination or 

 a turning of this part of the tube inside 

 out, as one turns the finger of a. glove; 

 this movement is quickly followed by the 

 ejection of the smaller part of the tube in 

 the same manner, by evagination. If the 

 body of some animal has touched 

 the cnidocil, then that body is pene- 

 trated by the thread-like tube, and 

 also possibly by a portion of the 

 larger tube with its recurved 

 hooks, and then the formic 

 acid of the capsule pours 

 into the tissues of the prey 

 and produces the general 

 paralysis above mentioned. 

 This paralysis, of course, is 

 not the effect of the formic 

 acid from one capsule, but 

 from many. Once used, the 

 capsule is useless, as the tube 

 cannot be withdrawn into it 

 again. 



Other tentacles also close 

 around the prey, and by 

 their combined action it is 

 conveyed through the mouth 

 into the general cavity ; here it may be seen, with microscopic aid, to break down and 

 go to pieces, the products of the disintegration being a fluid, evidently a nutritive one, 

 which then flows to all parts of the body, and the remnants of the hard chitinous skele- 

 ton which are ejected by the mouth or through an opening which may be extemporized 

 anywhere in the wall of the proboscis. This form of Hydra in which it is unconnected 

 with any other individual or zooid is termed the solitary condition. 



When the surroundings are favorable for its vegetative life, one usually may find 

 one or more Hydroe attached to the body of what appears to be a main stem or parent 

 form. These attached or appended zooids have been produced by a process of bud- 

 ding from the parent individual, and each one of them ultimately separates from its 

 parent by a constriction at its base and becomes a free and independent solitary 

 Hydra. A bud starts as a small, rounded swelling on the side of the body ; the swell- 

 ing being hollow, and its cavity being directly continuous with the general body-cavity 

 . of the parent ; by ordinary growth it attains considerable size, and from its distal end 

 a number of small swellings or prominences appear, which elongating, develop into 

 tentacles ; the portion of the bud anterior or distal to the tentacles becomes the pro- 

 boscis or hy][iostome, and a mouth is formed in its distal end. Being structurally com- 



FlG. 65. — Diagrams of onidocells; A, previous to emission of contents ; B 

 first stage of emission; C, filament completely extended; a, wall of 

 capsule; b, barbed sac; c, filament. 



