82 HYDEOZOA. 



to all parts. If, after having digested coloured prey, the polyp is made 

 to fast for some time, the vesicles gradually lose their deepened hue 

 and become comparatively transparent. The granules, therefore, would 

 seem to be specially connected with the absorption and distribution of 

 nutriment. 



(183.) When mature and well supplied with food, minute gemmules 

 or buds are developed from the common substance of the body ; they 

 spring from no particular part, but seem to be formed upon any portion 

 of the general surface. These gemmaB appear at first like delicate 

 gelatinous tubercles upon the exterior of the parent polyp; but as 

 they increase in size they gradually assume a similar form, become 

 perforated at their unattached extremity, and develope around the oral 

 aperture the tentacles characteristic of their species. 



(184.) This mode of propagation, termed " gemmation," differs from 

 the development of the Hydra ab ovo, inasmuch as the germ -cell, 

 whicH sets on foot the process, is derivative and included in the body 

 of the adult, instead of being primary and included in a free 

 ovum. 



(185.) Sometimes six or seven gemmae have been observed to sprout 

 at once from the same Hydra ; and although the whole process is con- 

 cluded in twenty-four hours, not unfrequently a third generation may 

 be observed springing from the newly-formed polyps even before their 

 separation from their parent : eighteen have in this manner been seen 

 united into one group ; so that, provided each individual, when complete, 

 exhibited equal fecundity, more than a million might be produced in 

 the course of a month from a single polyp. 



(186.) But perhaps the most remarkable feature in the history of 

 the Hydra is its power of being multiplied by mechanical division. If 

 a snip be made with a fine pair of scissors in the side of one of these 

 creatures, not only does the wound soon heal, but a young polyp sprouts 

 from the wounded part ; if it be cut into two portions by a transverse 

 incision, each speedily developes the wanting parts of its structure ; if 

 longitudinally divided, both portions soon become complete animals ; if 

 even it be cut into several parts, every one of them will in time assume 

 the form and functions of the original. 



(187.) It has recently been discovered that the Hydrce, at certain 

 seasons of the year, are reproduced from real ova*, at which period 

 various observers have proved them to be possessed of a male ap- 

 paratus of a most remarkable character. This strange organism makes 

 its appearance under the form of two, three, or four minute conical 

 tubercles, which become developed from the sides of the body, at a 

 short distance below the tentacula ; and in these, under the microscope, 



* " On a species of Hydra found in the Northumberland Lakes," by A. Hancock, 

 Esq., Ann. and Mag. of Nat. Hist, for 1850. 



