The Problem.

The problem under consideration may be briefly enunciated— “To what extent does direct development occur in echinoderms; what are its causes; by what processes does it take place; and what evidence does it provide of the evolutionary history of the echinoderms?”

In view of the more extensive nature of our knowledge of the subject in the Ophiuroidea, the present paper is predominantly concerned with that class, but wherever material is available comparisons are made with conditions holding in the other classes.