GENERAL ZOOLOGY 



of unfavorable conditions. The life cycle recorded for Epiphanes senta { Hydatina 

 senla), shown in Figure 12.2, indicates the complexity that is possible in 

 rotifer life histories. 



The tissues of rotifers are for the most part syncytial; that is, they con- 

 sist of large masses of cytoplasm containing numerous nuclei but without dis- 

 tinguishable intervening cell membranes. An interesting peculiarity of 

 rotifers, which they share with some other members of the phylum Aschel- 

 minthes, is that in each species the body contains a specific number of nuclei. 

 Furthermore, each organ contains always the same number of nuclei. 

 Normally, this constancy in nuclear number is established during embryonic 

 development, and none of the nuclei is capable of subsequent division. A 

 female, then, can deposit only as many eggs as there are germ cell nuclei in 

 the ovary at the time of maturation; there is no continuous process of 

 gametogenesis. The inability of the cells or nuclei of rotifers to divide 

 also limits the regenerative powers of the animals. Under experimental con- 

 ditions only very small parts can be successfully replaced by the animal — and 

 only parts which do not contain nuclei. 



Rotifers are important as animals with complex structure and life cycles, 

 and as animals which constitute a significant group in the organization of a 

 fresh-water environment. Classically, they have also been considered by 

 phylogenesists as of possible significance from yet another standpoint. The 

 rotifers exhibit a grade of organization roughly comparable to that of the 

 so-called trochophore larva, characteristically found in the developmental 

 cycles of many moUusks and annelids (pp. 375, 401). It should not be 

 thought that rotifers are "persistent trochophores" in the strict sense; rotifers 

 develop many extreme structural and functional specializations that are not 

 characteristic of trochophore larvae. In general, however, the Rotifera may 

 be thought of as representing a phylogenetic level of development which 

 might well have produced also the trochophore-like ancestors of the mollusks 

 and the segmented worms. This is an attractive theory, although the grounds 

 upon which it rests are tenuous at best; if correct, it could do much to 

 clarify the ancestral relationships between pseudocoelomate and eucoelomate 

 animals. 



The Class Nematoda. This class, from many standpoints the most 

 important group of the aschelminths, is known principally through its 

 parasitic members, which have long been familiar objects of study. Only 

 comparatively recently has attention been directed toward the many species 

 of free-living nematodes which occur in fantastic profusion in fresh water, 

 salt water, moist soil, and other habitats. The populations of microscopic 

 free-living nematodes inhabiting suitable environments are truly astronomical 

 in numbers of individuals. Many species of nematodes are free-living at all 

 stages in the life cycle, but perhaps the majority are parasitic at some time. 

 It is possible to list a series of 19 general types of life cycles for the nematodes, 

 ranging from the truly free-living forms with direct development, to a few 

 wholly parasitic species. Many are free-living in younger stages and become 



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