STRUCTURE OF THE LARVA OF SPONGILLA LAOUSTRIS. 471 



been fixed for approximately the same length of time as that represented in 

 section in Fig. 31. Like the pupa drawn in section in Fig. 31, it liad not 

 advanced far from the condition found to be characteristic of the second larval 

 type when first fixed. 



Fig. 24 represents an irregular portion of the marginal membrane, while 

 Fig. 24 a represents a portion which has already acquired an unbroken outline. 

 In both figures several yolk bodies (y. h) are seen, and it may be that there 

 is here and there an occasional nucleus of a flagellated cell. But nearly all 

 of tiiem are bodies of the same nature as tliose marked y. b. in Fig. 23. 



Fig. 24 h. — X 350. This figure represents a surface view of a portion of 

 the larva close to the inner limit of the marginal membrane. Tl'e nuclei of 

 the flagellated cells and the yolk bodies seen in the plasmodia! aggregations 

 are not easy to distinguish from one another (comp. Fig. 31). The small cells 

 derived from the cell groups, which are so characteristic of the larva described 

 as type B, are aggregated together in a loose fashion, and are proceeding to 

 develop into collar-cells. 



At the right lower corner the dermal membrane (D) alone, consisting of 

 flattened epithelium, has been drawn, a large subdermal cavity being situated 

 immediately below it. The developing chambers, i. e. the cell groups, are 

 somewhat loosely held together by cells with granular nuclei {con. cell). Tlie 

 nuclei of the flat epithelium — not seen in the same focus as those of the cell 

 groups — are drawn over or above the groups of chamber cells in the lower 

 part of the figure, but not in the upper part. The group in the right-hand 

 upper corner of the figure has been left clear without anything being drawn 

 above it (c. e., g. c). The two exceedingly large granular nuclei are possibly 

 nuclei of developing pore cells, which have already lost their vesicular 

 character {p. c. n.). 



Fig. 25. — X 1000. Portion of a section of a larva similar to that of which 

 a part is drawn in surface view in Fig. 23. The small cells in both figures 

 are the flagellated cells of the larva. The bodies enclosed in the plasmodial 

 aggregations are in some cases yolk bodies (y. b.), and in others nuclei of 

 flagellated cells {n.Jl. c); but the formation of the plasmodial stage is as 

 yet far from complete. Note that the nuclei of the small cells {fl. c.) are 

 much smaller than those of the cells c. c. in Figs. 24 b and 31. The latter 

 have already attained the structure of the collar- cell nuclei, while the former 

 are only in a stage of preparation for the formation of plasmodial aggregations. 



Fig. 26. — X 1000. Portion of a section of an individual slightly more 

 advanced than that drawn in Fig. 25, also derived from type D. This figure 

 should be carefully compared with Figs. 16 and 16 a. The plasmodial aggre- 

 gations {pi. a.) are fully formed, and by means of the differentiating stain the 

 nuclei of the flagellated cells are easily distinguished from the yolk bodies, 

 though there is no sign of cell outlines. 



