98 HYDBOZOA. 



membrane and the thin operculum of the ovarian capsule in which it 

 was formed. 



(234.) When the " female polyp" has thus escaped from the ovarian 

 vesicle of the common polypary, it has the appearance of a globular 

 transparent capsule attached by a short pedicle to the operculum 

 through which it has made its way, the orifice whereby it escaped 

 having closed around its stem. The tentacles are twelve in number ; 

 and from the circle surrounding their base, four canals may be traced, de- 

 scending in the substance of the globular body to terminate in the pedicle 

 or sacculus that occupies the lower part of the (Medusiform) capsule. 



(235.) On the rupture of the external membrane of the ovum en- 

 closed in the " female polyp," the young animal escapes, under a form 

 not at all resembling that of the parent animal. 



(236.) It presents at this period the appearance of a little worm, of 

 an elliptical shape, slightly flattened. Its entire surface is thickly 

 covered with vibratile cilia, by the agency of which it moves about even 

 while still imprisoned in the body of its mother, from which it subse- 

 quently makes its escape through the oral orifice. Generally each 

 " female" gives birth to two embryos, occasionally to three. 



(237.) No sooner has the young larva got free than it begins to 

 swim about, by means of its cilia, with a uniform gliding motion : 

 sometimes it turns round incessantly upon its axis, either horizontally 

 or in a vertical direction, at the same time varying its shape from that 

 of an egg to that of a pear. It is of a whitish colour, but still suffi- 

 ciently transparent under the microscope to show that it contains a cavity 

 filled with a coloured fluid, and composed of two membranes, whereof 

 one, the outer, is transparent as glass, the internal slightly opake. 



(238.) Repeated observations render it improbable that in this con- 

 dition the little embryo is nourished by means of a mouth. 



(239.) After swimming about for some time in the above condition, 

 the young creature fixes itself to some foreign body, such as a fucus, 

 or other marine production ; and then its form begins to be entirely 

 changed, and it is converted into a flat, circular disk, around which the 

 cilia, now quiescent, form a circular transparent fringe. In the centre 

 of its internal cavity an opake round spot makes its appearance, the 

 size of which is about a fifth part of that of the whole body, composed 

 of a mass of granules placed concentrically, and occupying the situation 

 whence the stem of the nascent polypary is to be developed. At this 

 point the external membrane becomes slightly thickened, and, as it 

 were, furrowed with vessels proceeding from the internal cavity. From 

 the opake central spot arises a hemispherical protuberance ; and at the 

 same time the central cavity loses its semicircular form and becomes 

 divided into four or five irregular lobes, which subsequently become 

 the horizontal supports of the fixed polypary. 



(240.) Already the whole expansion is covered with a horny layer ; 



