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of embryonization of development (Ivanova-Kazas, 1975), which is not the 

 case in the groups under discussion. Of course, the free-swimming blastula or 

 early gastrula of echinoderms differs in many features from the similar stage 

 of bivalves. The principal difference is the presence already at this stage in 

 mollusks of the rudiment of a shell gland, which is a. clear example of the 

 tendency towards "adultation" of development in bivalves (Jagertsen, 1972). 

 In echinoderms this tendency appears much weaker. As a resuh of "adultation" 

 the transition of larvae to definitive organisms has the nature of "evolutive" 

 metamorphosis in bivalves. For echinoderms, catastrophic metamorphism is 

 characteristic, which has caused greater divergence in the structure of pelagic 

 larvae and definitive organisms due to the changeover in the ancestral type 

 to radial symmetry and a sessile mode of life in the adult animal. 



Speaking of the parallel paths in the evolution of pelagic larvae of bivalves 

 and echinoderms, mention must be made of lecithotrophic larvae, in which 

 the ciliary cover serves mainly as a locomotory apparatus. The ciliary cover 

 in the doliolaria of sea cucumbers is quite identical with the cover of the 

 doliolaria of sea lilies and brittle stars; on the other hand, the ciliary cover 

 of these echinoderm larvae is analogous with that of the lecithotrophic larvae 

 of bivalves of the subclass Protobranchia. 



In conclusion, we would like to say a few words on some, in our opinion, 

 promising trends in investigation of the life cycles of bivalves and echino- 

 derms. The results presented in the preceding chapters together with the 

 material presented in our book "Razmnozhenie iglokozhikh i dvustvorchatykh 

 mollyuskov" [Reproduction of Echinoderms and Bivalves] (1980) provide an 

 overall picture of the life cycles of the investigated groups of animals in a 

 morphological aspect. Some answers have been obtained to the first questions 

 arising during studies on the life cycles of marine invertebrates with pelagic 

 larvae: when do the animals spawn? what processes occur in their gonads 

 throughout the year? which type of larva do they have? and, how to identify 

 this larva? All of this information is necessary but still insufficient for under- 

 standing how the actual life cycles proceed and which mechanism ensures 

 them. 



Morphological approaches to investigation of the life cycles, based on a 

 knowledge of the reproductive cycles occurring in the gonads on one hand, 

 and descriptions of larval morphology, on the other, may at the cytological 

 level explain the cytological peculiarities that ensure any variation of the life 

 cycle. However, the considerable data on gametogenesis and cytology of 

 larvae have not yet been examined from this viewpoint. 



Our book does not consider the ecological aspects of larval life. Numer- 

 ous investigations on the ecology of larvae in nature, conducted in the Sea 

 of Japan and various regions of the World Ocean have been supplemented in 

 the last decade with laboratory studies on the influence of ecological factors 



