v "'- U Ritter-Davis. — Enteropneusta. 197 



The cilia, by which alone the swimming movement is produced, 

 work in a perfectly invariable way so far as concerns direction 

 of stroke. The direction of the movement is determined by the 



orientation of the body, and the orientation is determined chiefly 

 at least, probably wholly, by the difference in specific gravity of 

 the two ends of the larva. Tins difference in specific gravity of 

 the two ends is due chiefly to the distribution of the coelomic 

 muss by which, as has been shown, the reduction of the specific 

 gravity as a whole is accomplished. The anterior end of the 

 larva is, during all the first and second part of the third stage, 

 directed upward, quite vertically at times, but more frequently at 

 a small angle with the vertical. The stroke of the cilia being, 

 then, backward, the body is of necessity driven in general 

 upward. 



Concerning the specific gravity of the two ends, it will be seen 

 from our account of the structure of the larva, and by reference 

 to PI. XVII, Figs. 1 and 2, that while the posterior end is some- 

 what broader than the anterior, and hence might from this fact by 

 itself contain the larger portion of the coelomic mass, a distinctly 

 greater proportion of the anterior end is occupied by the mass 

 because of the absence from this end of any portion of the 

 digestive tract. This tract contributes to the special reduction 

 of the specific gravity of the anterior half in two ways: first by 

 making room there through its absence for the larger portion of 

 the coelomic mass, and second by its making in the posterior half 

 the larger portion of the protoplasm of the larva. Not only, it will 

 be observed, is nearly all of the voluminous digestive tract in the 

 posterior half, but both pairs of mesoblastic vesicles are not 

 only here, but are situated far back. PI. XVII, Pig. 3, shows 

 the position of these mesoblasts in Tornaria huboardi, but the 

 same fact would have appeared in quite as pronounced a way 

 in T. ritteri, had the mesoblasts been shown in Figs. 1 and 2. 

 Attention should be called also to the extremely small size 

 of the single interior mesoblastic vesicle during the first and 

 second larval periods. 



When stating the changes occuring in metamorphosis in 

 their true sequence, we have emphasized the fact that the 

 reduction in size occurs first at the anterior end, and that 



