INTRODUCTION 



The life cycles of commercially exploited and cultured mollusks and echino- 

 derms comprise, in most cases, a prolonged benthic period and a brief pelagic 

 period. In the benthic period the animal feeds, grows, attains sexual maturity, 

 produces gametes, and participates in reproduction. The prodijction of gametes 

 (gametogenesis) and their subsequent release and fertilization by mollusks and 

 echinoderms inhabiting temperate waters usually occurs every year. The an- 

 nual reproductive processes in bivalves and echinoderms inhabiting the Sea of 

 Japan were described earlier by researchers in the Laboratory of Embryology, 

 Institute of Marine Biology [Razmnozhenie iglokozhikh i dvustvorchatykh 

 mollyuskov (Reproduction of Echinoderms and Bivalves), Kasyanov et al., 

 1980]. The present book is a continuation of the 1980 publication. Here we 

 describe the larvae of bivalves, sea stars, sea urchins, brittle stars, and sea 

 cucumbers of the Sea of Japan, which are economically important or abundant 

 forms in benthic communities. The larvae of these species are planktonic, feed 

 on phytoplankton, grow, and are carried out to sea by currents. Then they 

 migrate froni the surface to the bottom, settle there, and metamorphose, thus 

 completing the pelagic period of their life cycle. This period is less known and 

 understood than the benthic, even though many papers and books deal with the 

 subject. Among such works, mention should be made of those by Thorson 

 (1936, 1946, 1950), Mortensen( 1921, 1937, 1938), Mileikovskii (1977, 1981), 

 and Jagersten (1972). The structure of the larvae discussed here have been 

 examined by O.M. Ivanova-Kazas in her two-volume work on the embryology 

 of mollusks and echinoderms (Ivanova-Kazas, 1977, 1978). 



Particular attention is paid in the present book to those characters which 

 make possible the determination of the species affinities of larvae. Thus 1 8 

 species of bivalves and 10 species of echinoderms are described in detail, and 

 keys are provided to enable identification to family level of planktotrophic 

 larvae of bivalves and echinoderms from Peter the Great Bay. The descriptions 

 of the larvae are preceded by descriptions of the morphology, physiology, and 



