The origin and phylogenetic significance of the trochophoran larvae. 2. Evolutionary significance of the larvae of coelomate worms and molluscs

Ivanova-Kazas, OM.

Trochophore is the main type of larvae in the Polychaeta, Echiurida, Sipunculida, and Mollusca but larvae of some other types are also described in these groups, thus demonstrating the possibility of important evolutionary modifications at the larval stage. The atrochous type of larva, characteristic of some polychaetes, is to be considered as a primitive one. The prolongation of pelagic life caused differentiation of the ciliary system, complication of the internal structure, and, thus, development of trochophoran larva. It is not excluded that trochophore originated independently in different groups. Metatrochophore, pelagosphere and veligar are transitional stages of development which combine both purely larval and adult characters. The larva of endolarval type is regarded as a secondarily modified trochophore.