582 



FRESH-WATER BIOLOGY 



and the species is carried over the winter in the resting eggs re- 

 sulting from fertilization by the males. In the winter rotifers, the 

 males appear in the spring, and the species is carried over the sum- 

 mer by the resting eggs. 



By the greater number of rotifers the eggs are laid as soon as 

 they are completely formed, development taking place outside the 

 body of the mother. But some rotifers are viviparous, the egg 

 being retained in the mother's body until it is partly or completely 

 developed. It is remarkable that the viviparous condition is found 

 in several different groups of rotifers that are not closely related, 

 so that it must have been developed independently several times 

 within the Rotifera. Asplanchna and Rotifer are among the best- 

 known viviparous genera. Philodina and Callidina, closely related 

 to Rotifer, as a rule deposit the eggs undeveloped, though certain 

 species in both these genera produce living young. Thus ovi- 



parity and viviparity, which in 

 some higher animals distinguish 

 grand classes, are among the 

 rotifers both found in the limits 

 of a single genus. 



It is a rather remarkable fact 

 that the cleavage and early de- 

 velopment of the rotifer egg does 

 not resemble that of the animals 

 to which the rotifers have often 

 been considered the nearest rela- 

 tives. In annelids and lower 

 mollusks the early development 

 shows a remarkable similarity 



9" 8 . 6 9-.. Developing egg of a rotifer' Asplanchna CVCn in the details of the Spiral 

 nernckit de Guerne. a, Single cell stage; b, four 



cells; c, twenty cells; d, ninety-four cells; e, optical cleavage. But in the TOtiferS 

 section through embryo formed of many cells. 



the cleavage follows a completely 



different type (Fig. 869). The developing rotifer forms a solid mass, 

 which contains no cavity until the organs formed within this mass 

 begin to separate, just before the rotifer takes its final form and 

 becomes active. 

 When living the body of the Rotifera is usually transparent and 



FIG 



