CONCLUSION 



The main purpose of this book has been to present the bulk of factual material 

 which requires further observations and analysis. In concluding it, perhaps a 

 brief comparison of the larvae of bivalves and echinoderms is warranted. To 

 a significant extent independent of the phylogenetic affinity of the organisms 

 and the similarity of the ecological niches occupied by them, the planktotrophic 

 larvae of these animals are highly similar in morphology, physiology, and 

 behavior. Such a similarity is the result of adaptation to a pelagic mode of 

 life. In contrast to the diversity of ecological conditions in the benthic period 

 of their life cycle, these organisms experience in the water column a more 

 uniform habitat or environment. The most important adaptation of larvae to 

 swimming and feeding in water was the appearance of the ciliated band — 

 the principal multifunctional provisional organ, which is primarily a locomo- 

 tory and trapping apparatus. The function of the ciliated band also facilitates 

 the processes of respiration and excretion and its separate elements serve as 

 sensory organs. The location of the ciliated bands on various processes of the 

 larval body (arms, processes, velum) facilitates floatation of the larvae; these 

 processes often function as stabilizers and rudders. The characteristic defen- 

 sive feature of pelagic larvae, as also of other planktonic organisms, is their 

 relative colorlessness and transparency. There are also some other characters 

 ensuring for planktotrophic larvae life in the water column. Some of these 

 features can be traced directly from the larvae of primitive multicellular 

 creatures, while others arose secondarily during the evolution of various 

 groups. 



Comparing larval periods in the life cycles of bivalves and echinoderms, 

 we note that the postembryonal larval period begins exceptionally early, from 

 the stage of the late blastula or the early gastrula. Such an early transition to 

 the postembryonal period is encountered in lower multicellular animals, for 

 example hydroid polyps, but in most other animals it is delayed as a result 



