But further, in the class from which I have taken this last illustration--that of the ROTIFERA--there are examples of viviparous genera; and these, because of the perfect transparency of all the integuments, are peculiarly instructive and germane to my argument. In _Rotifer macrurus_ the ovary with its germinal vesicles is distinctly seen occupying one side of the animal. From this one of the vesicles enlarges, until it becomes a long-oval translucent sac, nearly filling the whole left side of the visceral cavity. A kind of spasmodic movement is suddenly observed in this oblong ovum, and instantly we see, in its place, a well-developed living young; as distinctly visible as if it were excluded. It lies in a bent position, with its foot upturned; is nearly half the length of the parent; is furnished with a proboscis, with a pair of crimson eyes, with ciliary wheels, with a mastax whose toothed hemispheres frequently work vigorously, and with all the viscera proper to the species. [Illustration: PREGNANT ASPLANCHNA. _a._ _Unborn young._] In the beautiful, comparatively large, and economically singular genus, _Asplanchna_, the same process of development can be watched with perfect facility through every stage. In the body of the female parent, as transparent as the clearest glass, the band-like ovary is seen floating in the visceral cavity, with several ova in various degrees of advancement. We trace one of these till it becomes a manifestly living young in the ovisac, lying along at the bottom of the parental cavity, more than one-third of whose volume is occupied by it:--supposing it to be a female infant. All its organs,--the eyes, the jaws, the stomach, the pancreatic glands, _the ovary with its nuclei_, the muscles, the rotatory cilia, &c. can be traced with the utmost distinctness long before birth, and its motions are strong and voluntary. Neither in this case, nor in that of _Rotifer_, does the young animal pass through any metamorphosis; the unborn young has the full development of the parent, in every respect but size. In each case, the _visible_ life-history of the individual commences not at birth, but at a period long antecedent, if indeed it can be said to commence at all, where we see it gradually developed from a nucleus, which was an integral part of the parental ovary, _even before that parent's birth_. In the case of the amusing little Water-fleas (_Daphnia_), we have another example of viviparous generation, which, owing to the same cause as in the ROTIFERA,--the transparency of the integument, can be followed through all its stages by the eye of the observer. The eggs of this little Crustacean are deposited in a special chamber within the valves of the parent, where they are hatched. The young remain in their receptacle for a period, which varies according to the temperature, but long enough for them to undergo important changes in structure, and to pass their first moult.[96]