HYDRA. 



[ 329 ] 



HYDRA. 



sidered as doubtful and requiring further in- 

 vestigation. In addition to these stinging 

 organs, we have found other very minute 

 capsules (fig. 22 c), containing a filament 

 curved even when emitted, the nature of 

 which is obscure. 



A third kind of organ is said to have been 

 met with also in the surface of the body, 

 consisting of ovate capsules or bodies, from 

 ■which a stout and short filament projects. 

 These appear to resemble the organs of ad- 

 hesion of the Acalephse ; but as their size is 

 not stated, nor the diameters of the figures ex- 

 pressed, we have been unable to identify them. 



The body and tentacles of Hydra are hol- 

 low. The prey, which consists of Entomos- 

 traca, small Annulata, &c., when caught by 

 one or more of the tentacles extended for 

 the purpose, is slowly brought to the mouth, 

 and forced into the cavitv of the bodv, in 

 which it is digested ; the undigested portions 

 being evacuated through the mouth. It is 

 still a question whether a posterior outlet to 

 the cavity of the body exists. The posterior 

 part of the body is more or less dilated into 

 a flattened disc, which, by its suctorial power, 

 enables the animal to attach itself to various 

 bodies. Hancock has seen excrementitious 

 matter passing through the body at this 

 l)art and the disc ; but most, if not all, pre- 

 vious observers have denied the existence of 

 a canal. The cavities of the tentacles have 

 been described as containing a semifluid sub- 

 stance, undergoing a kind of circulation; 

 and the cavity of both the tentacles and the 

 body have been stated to be lined with cilia. 



The extraordinary power which Hydra 

 possesses of reproducing lost parts is truly 

 wonderful. Thus, if the body be cut into 

 two or more, or even forty parts, each con- 

 tinues to live, and developes a perfect new 

 animal. If the section be made lengthwise, 

 so as to divide the bodv all but the end, the 

 tv/o portions become resoldered and form a 

 perfect being ; if the pieces be kept asunder, 

 each becomes a Hydra, the two possessing 

 but one posterior end; if the section be 

 made from the tail towards the head, the 

 two bodies will be perfected and remain 

 attached to the one head. If a tentacle be 

 cut off, a new animal is formed from it. 

 When one end of the body of a Hydra is 

 introduced into the body of another, the two 

 imite and form one. The head cut off one, 

 may be engrafted upon the body of another 

 which wants one. And when the body is 

 turned inside out, the outer surface which 

 has thus become the inner will perform the 



ordinary digestive functions, and the animal 

 will continue to live. 



The ordinary mode of reproduction of 

 Hydra is by gemmation ; a minute swelling 

 forms upon some part of the surface of the 

 body, this enlarges, and gradually assumes 

 the form of the parent, while remaining 

 attached to it. Sometimes several of these 

 are formed upon a single individual at the 

 same time, and so, remaining adherent, they 

 give the animal a branched appearance (PI. 

 33. fig. 21). 



At certain seasons of the year, as at the 

 end of summer or in the autumn, reproduc- 

 tion takes place by the formation of sperma- 

 tozoa and ova. The spermatozoa are formed 

 within spermatic capsules. These arise as 

 minute conical tubercles a little beneath the 

 base of the tentacles, one on each side (PI. 

 33. fig. 24 a) ; and the spermatozoa are libe- 

 rated from them by bursting. The sperma- 

 tozoa in the figures resemble those of the 

 Mammalia, except that the tails are undu- 

 late. The ova are fm-nished with a thick 

 coat, and are formed in the substance of the 

 lower part of the body (fig. 24 6). They 

 subsequently separate from the body, and 

 appear to be capable of spontaneous motion, 

 but whether from the presence of cilia or 

 not is undecided. The sac of the ovum then 

 becomes ruptured, and the new animal es- 

 capes (fig. 25). 



Hydrce are ver}^ common. The best me- 

 thod of procuring them is to collect a number 

 of water-plants from any clear pool or slow 

 stream, and bring them home in an india- 

 rubber bag (sponge-bag). On placing the 

 jilants subsequently in a glass jar (con- 

 fectioners' jar) containing water, they will be 

 found at the end of some hours with the 

 tentacles fully extended in search of prey, 

 when they are easily recognized. They 

 usually adhere to the sides of the glass, or 

 to the stems or under sides of the leaves of 

 the plants; but sometimes they are seen 

 suspended from the surface of the water by 

 the sucker, which is protruded just above it 

 so as to become partly dry. A number of 

 small Entomostraca should be added to the 

 water, as they are very voracious. 



Some of the species of Hydra are occa- 

 sionally covered with minute parasitic Infu- 

 soria, viz. Kerona polyporum {Pi. 41. fig. 13), 

 which is found upon H. vulgaris and fusca ; 

 and Trichodina pedicnlus (PI. 24. fig. 16), 

 which occurs upon H. vulgaris and viridis. 

 It is an interesting sight to see these running 

 up and down the tentacles and surface of the 



