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 ; and it is a remarkable 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 Acaridce 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 far 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 Tunicata 

 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 



