STRUCTURE OF THE LARVA OF SPONGILLA LACUSTRIS. 415 



The larval cavity is not obliterated^ but takes part in the for- 

 mation of the exhalant system and gastral cavity. 



(2) Special Features of Type D during Metamor- 

 phosis. — There remain but a few statements to make with 

 regard to the metamorphosis of the type of larva now under 

 consideration, owing to the completeness of the description 

 given above of the features common to the metamorphosis of 

 all the larvae. 



The interchange of position between the flagellated cells on 

 the one hand, and the cells with granular nuclei on the other, 

 takes place much in the same way as in the larva described as 

 type C. There are two features, however, which are perhaps 

 worthy of further notice : first, the flagellated cells migrate 

 almost at the same rate from the upper and lower surfaces ; 

 and secondly, they tend to form fan-like groups at the lower 

 surface. 



In fig. 15 only a few plasmodial aggregations have been 

 formed, but notwithstanding this fact the flagellated cells have 

 disappeared from both surfaces, and the flattened epithelium 

 is complete almost everywhere. The arrangement of the cells 

 is, therefore, quite diff'erent from that represented in fig. 29, 

 which has been drawn from a pupa derived from the larva 

 described as type C. In figs. 15 a and h the second point, 

 namely, the formation of fan-like groups, is illustrated. The 

 groups in question may in some cases consist of so many cells 

 as to make it possible that an actual invagination has taken 

 place, though for other reasons this is not probable (fig. 15 h\ 



The individual cells of the groups in question appear as if 

 they had been drawn inwards by some internal force, the 

 body of the cell being in consequence greatly elongated 

 and attenuated. The dark streak which was described above 

 as being situated externally to the nucleus of the flagellated 

 cells in the free-swimming larva is seen clearly in many 

 of the cells contained in the groups, though there is no 

 sign whatever of the portion of the flagellum that lay out- 

 side the cell. The nuclei of the flagellated cells do not appear 

 to change in character so long as the cells remain in these 



