STRUCTURE OP THE LARVA OF SP0NC4ILLA LAOUSTRTS. 419 



are found in the spaces between the chambers where tlie latter 

 are always situated. In these positions, especially in the 

 neighbourhood of the developing skeletal fibres, they form a 

 sort of connective tissue, and the spongin occurring in the 

 sponge seems to owe its origin to these cells. Wherever a 

 canal is formed, it is, of course, lined by these cells. The cells 

 with vesicular nuclei also frequently take up a position adjacent 

 to the cauals. Whenever they do so the character of their 

 nuclei is doomed to change just as the nuclei of the cells 

 which give rise to the megascleres become modified. 



It makes no difference, so far as the various classes of cells 

 are concerned, whether the gastral cavity is a new formation, 

 as in the young sponge developed from a larva of type D, or is 

 derived mainly from the larval cavity, as was shown to be the 

 case in the development of the larva of type C. In each case 

 the cavity is lined by the same class of cells, that is, by those 

 with granular nuclei. This fact does away with any difficulty 

 that might arise from the histological point of view. 



The cells with vesicular nuclei, after the flagellated cells 

 have separated themselves from them and have become ar- 

 ranged in chambers, remain in the neighbourhood of these 

 structures and constitute the amoeboid elements of the young 

 sponge. They often adhere to the surface of the chambers, 

 being in many cases situated at a point not far from the 

 apopyle as well as elsewhere, and to a certain extent spread 

 over them. It is quite possible that some of them in this 

 position become perforated to form the openings of the in- 

 halant canals into the flagellated chambers. This point is 

 further explained in the descriptions of figs. 32, 33, and 34, 

 in which inhalant pores are figured. It is difficult to prove 

 that this pore is intra-cellular because of the great superficial 

 expansion of the cells in the neighbourhood of the pore, and 

 the distance which, as a consequence, intervenes between the 

 nucleus of the cell and the pore. If these pores are really 

 intra-cellular, then the cells in question would correspond to 

 the porocytes which have been described by Mr. Minchin in 

 the Ascons. The only explanation I can give of the appear- 



