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region of the interm ediate cells is generally m arked by a slight constriction, giving a waist, as it were, to the larvae. The granular cells are much fewer in num ber than the other elements, and are also of m uch larger size, but there are gradations in this respect, those placed at the posterior pole being much larger than those which border upon the interm ediate cells.
During the free-swimming larval period, considerable changes take place in the relative proportions of the different parts of the larvae. In the newly hatched larva (fig. I) the anterior ciliated region is relatively large, w ith a very broad granular border to the cells, and the posterior granular cells are few in number. The number of granular cells now increases at the expense of the ciliated cells. Some of the ciliated cells, by absorption of the internal retractile portion of the cell, become interm ediate cells, and these, in their turn, absorb their flagellum, increase in size, and become granular cells. This process goes on pari passu w ith a decrease in the granular border of the ciliated cells. Tn the larva of about twenty-four hours (fig. 2), the granular cells form a mass equal to th at of the ciliated cells, and the latter have now a very narrow granular border. In
