STUUCTUEE OF THE LARVA OF Sl'ONGILLA LACUS'l'iaS. 385 



flagellated cells. These considerations suggest irresistibly the 

 conclusion that they are developed from cells identical iu 

 character with those which gave rise to the flagellated cells, 

 and possibly in the same way (figs. 5, 5 a — c, 9 a, b). 



I have no reason for thinking that this type of larva often 

 fixes itself, though it may occasionally do so. As a rule, how- 

 ever, it develops further, and gives rise to the larva of type 

 C, the special features of which will be described next. 



Special Features of Type C. 



In this type of larva the same three kinds of cells are found 

 as in type B, but, owing to the differentiation and development 

 which have taken place, the differences of the cell character- 

 istics are considerable. 



The cells with granular nuclei are much more irregular in 

 shape than in the larvae described above. In this larva they 

 branch extensively, and unite by their processes to form mem- 

 branes which are still more or less incomplete, both under the 

 flagellated layer and as a lining to the larval cavity. Many of 

 those which are situated in the interior have changed con- 

 siderably in shape, having in many cases flattened out so as to 

 surround small lacunar spaces or canals (fig. 14 a). The 

 lacunae in question are destined to become the exhalant system, 

 and the cells surrounding them to become the flat epithelium 

 of the same. Simultaneously with the flattening of the cell 

 body the shape of the nucleus is also changed to that of a bi- 

 convex disc. In this type some of the cells with granular nuclei 

 may develop a spiny microsclere (fig. 37 a). 



It is necessary to point out here that the spaces and canals 

 lined by the cells with granular nuclei are not in any way 

 comparable to the spaces surrounded by the cell groups. In 

 the latter case, the spaces are the cavities of future chambers 

 which are either appearing or have already appeared iu the 

 larva. The small spaces and short canals here described are 

 lacunar cavities which have the same relation in many cases 

 to the cavities of the cell groups as the exhalant canals have 



