434 Mr. E. W. MacBride. [Dec. 7, 



and is completely separated from the hydroccele. The just fixed larva 

 in both cases we take to represent the ancestor of Echinoderms, jnst 

 after it had given up its free-swimming life (fig. 2). The curious, and 



FIG. 2. Supposed Ancestor of Asterids and Crinoids. 



as yet unexplained, peculiarity of Echinoderms, the predominance of 

 the left side (left hydrocoele and left posterior "body cavity), soon 

 made itself felt. Starting from this point, however, ontogeny plainly 

 teaches us that Asterina and Antedon have diverged in two opposite 

 directions. In Antedon an excessive growth of the ventral surface 

 has rotated mouth and hydrocoele backwards and upwards away 

 from the stalk (fig. 3). A precisely similar change to this takes 

 place, as we know, in Oiona and Pedicellina, and it is to place the 

 mouth in a favourable position to catch pelagic prey. In Asterina, on 

 the other hand, the body is flexed ventrally on the stalk (fig. 4), so 



