576 . DEVELOPMENT OF ROTIFERA AND ARACHNIDA. 
rate ; so that from the known rate of propagation in a 
Hydatina, it is calculated that nearly seventeen millions might 
be produced from a single individual within twenty-four days, 
although not more than three or four ova are being brought 
forward at once. The latter takes place only at certain 
seasons, most frequently before the winter; the sexual eggs, 
in these as in other cases, being endowed with a special power. 
of resisting cold. It is only occasionally, therefore, that 
males are to be met with ; andit is aremarkable circumstance 
that they are in many instances destined for so brief an exist- 
ence, as not even to be furnished with any digestive apparatus ; 
so that their development is completed, and their reproduc- 
tive function performed, at the sole expense of the nutriment 
which was furnished by the egg. 
751. In the class of Arachnida there is nothing that corre- 
sponds with the metamorphosis of Insects ; for Spiders and 
Scorpions attain to the full development of their organs within 
the egg, so that the young come-forth from it differing in 
scarcely any respect from their parents, except in size; and 
among the Acarid@ or “ mites,” the only important difference 
lies in the deficiency of one of the four pairs of legs, which is 
supplied after the first moult. This completion of the process 
of development within the egg could scarcely take place, but 
for the large supply of nutriment afforded to the embryo 
by the yolk. The Arachnida deposit a fai smaller number of 
eggs than Insects do; and thus, as each egg can be made of 
much greater size, there is no necessity for that early emersion 
of the embryo in search of the material for its continued de- 
velopment, which has been shown to be the real purpose of 
the larva-life of Insects (§ 745). 
752. In the Molluscous series the generative function pre- 
sents few such peculiarities as have been noticed among Articu- 
lata. Save in the lowest members of the series, the Z’unicata 
and the Polyzoa ($$ 114, 115), we have no example of repro- 
duction by gemmation ; and no instance of reproduction by 
agamic ova is yet known. The union of the two sexes in the 
same individual is much more common in this series than 
among Articulata; and thus the fertilization of the eggs is 
secured without the exercise of locomotive power. It is 
remarkable, however, that the embryos even of such Mollusks 
as are destined to remain almost motionless when they attain 
