84 HYDEOZOA. 



(fig. 40, l, a) ; when mature, this ovum becomes detached from the 

 parent animal and fixes itself to some foreign body. 



(189.) The ovigerous capsule is, when fully developed, of such a size 

 as to be seen with the naked eye. It is attached to the side of the 

 polyp, nearer to the foot than the spermatic capsules, and is distin- 

 guished from the rest by its spherical form and yellowish-brown colour. 

 In the Hydra viridis only one of these ova appears to be developed on 

 the body of the polyp at the same time ; but a number, varying from 

 five to seven, have been occasionally observed upon the Hydra fusca. 



(190.) The ovum appears, at first, as a small granular mass in the 

 thickness of the wall of the animal. As the spherical yelk-mass en- 

 larges, it projects from the side, seeming to carry along with it the 

 outer or clearer layer of the animal's body ; then the cells of this layer 

 grow thinner, and recede from the outer covering or capsule enveloping 

 the egg-like mass, which at the same time becomes much thicker, and 

 is left attached to the animal only by a narrow portion or pedicle. As 

 the development proceeds, a similar atrophy of the cells of the pedicle is 

 followed at last by the separation of the spherical mass, which is thus 

 detached from the parent polyp. 



(191.) From various observations it would seem that while some of 

 the individuals of the Hydras are hermaphrodite, others produce the 

 organs of one sex only ; but generally both kinds are developed from 

 the same Hydra. 



(192.) The fertilization of the ova cannot take place until after the 

 rupture of the spermatic cyst and that of the ovisac also ; so that the 

 parent has no more participation in it than has the Fucus in the ana- 

 logous fertilization of its germ-cells after their discharge. Although 

 the production of a new Hydra from such an egg has not yet been wit- 

 nessed, there seems no reason to doubt the fact. It would seem that this 

 alternation of reproduction between the gemmiparous and the sexual 

 is greatly influenced by temperature, the eggs being produced at the 

 approach of winter and serving to regenerate the species in the spring, 

 while the budding process naturally takes place only during the warmer 

 seasons of the year, but may be made to continue through the whole by 

 a sufficiently high temperature. 



(193.) Nearly related to the Hydrce is the remarkable group of Gela- 

 tinous Polyps, as they are named by Cuvier in his classification of the 

 Animal Kingdom, constituting the genus CORYNE (Corine of Gaertner). 

 One of these little animals, seen with the naked eye, observes Mr.Gosse*, 

 to whom we are indebted for the following particulars of their history, 

 looks like a very slender branching plant. It is altogether about as 

 thick as fine sewing-cotton, creeping along a frond of sea- weed, or other 

 substance upon which it grows, like an irregularly-winding thread. This 

 creeping root sends off frequent rootlets, which, crossing each other, 

 * Rambles on the Devonshire Coast. 1853. 



