STRUCTURE OF THE LARVA OF SPONGILLA LAOUSTRIS. 377 



body situated close to the nuclear membrane, seems to spread 

 over the membrane in question, which appears to be drawn out 

 like the outer coat of an onion. The internal part of the 

 flagellum can be traced along its whole length in all the larval 

 stages, as well as in some young fixed stages in which the 

 flagellated layer has not completely migrated into the interior 

 (figs. 29 a and S8a—d). 



The nucleus of the flagellated cell, when cut tangentially to 

 the surface of the larva, presents a circular section nearly 2 ^ 

 in diameter, but when cut radially it has an oval appearance, 

 or, perhaps more correctly, that of the outer half is almost 

 cone-shaped, while that of the inner half is semicircular. It 

 measures about 2 /x across and 2| fx in length. By combining 

 these two sections it is evident that the shape of the nucleus 

 is that of a cone with a hemisphere at its base, or, in other 

 words, it is onion-shaped (figs. 1 and 29 a). 



I must here apologise for going still further into detail con- 

 cerning the flagellated cells. This is necessary, however, in 

 order to compare the cells here described as '' cell groups " 

 with the flagellated cells. 



The cytoplasm of the flagellated cells is clear, and contains 

 at most no more than three or four small round granules, 

 which cannot be seen in ordinary preparations^ (figs. 38 a — d). 



The nucleus has a thicker and better defined nuclear mem- 

 brane than any other class of nuclei found in the larva. In- 

 ternally the nucleus contains a few small and irregularly shaped 

 chromatin granules, one of which may exceed the others in 

 size, scattered at the nodes of a somewhat coarse nuclear re- 

 ticulum. When there is one larger granule, as is often the 

 case, it usually occupies a central position, and the threads 

 pass straight from it to the nuclear membrane ; but when there 

 are several granules approximately equal in size, which is the 



* These small granules could not be seen in sections of larvae preserved 

 either in Fleramiug's fluid or in Perenji's fluid, or in absolute alcohol, and 

 mounted in Canada balsam. However, they were distinctly seen in prepara- 

 tions of larvse preserved in osmic vapour, stained in picro-carmine, and 

 mounted in glycerine. 



