370 RICHARD EVANS. 



and D, type B being an intermediate stage between types A and 

 C. Type D is not necessarily younger than type C. In the 

 development of type C the vesicular cells (j3) give rise to the 

 "cell groups" (y) rather than to the cells with granular nuclei 

 (a). In the development of type D the exact opposite takes 

 place. The relations can be expressed graphically by the fol- 

 lowing diagram : 



B 

 C 



D 



(6) Metamorphosis and Subsequent Development. 

 — The larva fixes itself either by the anterior pole, or by a 

 point not far from that pole, and these differences in the fixa- 

 tion are the causes of transitory variations in the form of the 

 newly fixed stages. In the case of type C the larval cavity 

 becomes obliterated soon after fixation. In the case of type 

 D, however, it seems practically certain that the larval cavity 

 is not obliterated. This may be inferred from the fact that a 

 cavity of considerable size is often found persistent in speci- 

 mens with fully formed flagellated chambers, with the flagel- 

 lated layer almost absent from the surface, with the flattened 

 epithelium almost complete all over, and with the developing 

 exhalant canals in some cases already opening into the 

 cavity. 



The flagellated cells pass into the interior either individually 

 or in groups, which present a fan-like appearance, and which 

 in some cases consist of a great number of cells (figs. 14, 15, 

 and 15 a and b). They pass in more rapidly on the lower than 

 on the upper surface, and this difference is more accentuated 

 in a larva of type C than in one of type D (comp. figs. 15 and 

 29). Simultaneously with the passing in of the flagellated 

 cells, the cells with granular nuclei pass out to form the 

 flattened epithelium of the upper and lower surfaces, as well 

 as to form the marginal membrane. The cells with granular 



