446 



ROTIFEEA. 



successively selected for development after the following manner : One 

 of the nuclei situated near the surface of the ovary attracts around it a 

 small portion of the granular protoplasm, which becomes detached from 

 the remaining contents of the ovary. The portion thus specially isolated 

 gradually enlarges, assuming at the same time a darker hue, and the 

 nucleus slightly enlarges, while its central nucleolus appears to become 

 absorbed. When the ovum, thus separated from the ovarian proto- 

 plasm, has attained its full size, it becomes invested by a thin shell, 

 which is apparently a secretion from its own surface. The ovum being 

 thus ready for expulsion (fig. 227, 1, q), is slowly forced down to the 

 lower part of the ovary, and, sweeping round the inferior border of the 

 lower stomach, passes through the dilated oviduct (fig. 227, 1, 1} and 

 enters the cloaca, whence, by a sudden contraction, it is expelled. 



(1156.) At this point of development the yelk 

 consists of a single segment ; but very soon the 

 central nucleus becomes drawn out and divides 

 into two, this division being followed by a corre- 

 sponding segmentation of the yelk. The same 

 process is repeated over and over again, until at 

 length the yelk becomes converted into a mass of 

 minute cells. The first trace of further organiza- 

 tion which presents itself appears in the form of 

 a few freely-moving cilia ; these are developed 

 at two points, one at c a, fig. 231, 1, which corre- 

 sponds with the future head, and the other near 

 the centre of the ovum (6), which is destined to 

 become the cavity of the stomach ; shortly after 

 this appearance of cilia, traces of the dental ap- 

 paratus become recognizable, this, again, being 

 soon succeeded by the union of the entire mass of 

 yelk- cells, and the formation from them of the 

 various organs of the animal. The cilia now play very freely, especially 

 at the head (a) ; the creature twists itself about in its shell ; and two 

 red spots (c c), regarded by Ehrenberg as organs of vision, appear. The 

 young animal now bursts its shell, and presents the appearance repre- 

 sented in fig. 231, its whole organization, though obscurely seen, being 

 that of the perfect animal, and not of a larval state. 



(1157.) The young Melicerta, when first hatched, is free, and swims 

 about actively in the water for a short period, when, attaching itself 

 by its caudal extremity to some foreign object, it proceeds to manufac- 

 ture for itself a tube for its future residence by means of a most 

 remarkable apparatus appointed for the purpose. This is the append- 

 age called by Professor Williamson the fifth rotatory flap (fig. 227, 1, c), 

 and named by Mr. Gosse* the "chin" or "pellet-cup" in which the 

 * Quarterly Journal of Microscopical Science. 



Young Melicerta. 



