STKUCTUEE OF THE LARVA OF SPONGILLA LACUSTRIS. 389 



same relation to the exhalaut pore already described as they 

 do ill the fully metamorphosed larva or adult sponge. The 

 cells have a horseshoe arrangement when seen in a radial section 

 of a group. The opening, which points towards the lacunar 

 spaces already mentioned, is a true exhalaut pore or apopyle 

 (fig. 7 a, B). 



The nucleus of the collar-cells is but very slightly larger than 

 it was in the incompletely divided condition seen in type B. 

 In the interior the chromatin granules have become more 

 numerous, and the nuclear reticulum more complicated. Con- 

 sequently there is at present no very striking difference between 

 these nuclei and some of the nuclei of the cells which have 

 been described as having granular nuclei. But for the dis- 

 covery of the origin of these cells in the larvse of type B, their 

 existence as a separate class would probably have remained un- 

 discovered and even unsuspected, and they might easily have 

 been placed among the cells with granular nuclei — an error 

 which would have almost certainly led the way to another 

 mistake, that, namely, of describing the cavities of the cell 

 groups as incipient exhalaut canals. 



So important are the main features of this type that they 

 may be briefly summarised. In the first place there are two 

 kinds of cavities, those of the flagellated chambers on the one 

 hand, and the lacunar spaces of the incipient exhalaut system 

 on the other hand. In the second place these two kinds of 

 cavities are surrounded by two different kinds of cells, — the 

 former by the individual elements of the cell groups or collar- 

 cells, and the latter by cells with grar^ular nuclei or flat epithe- 

 lium. The two cavities above described are, in certain cases, 

 already in communication by means of the exhalaut pore. 



The important point to emphasise is the fact that the 

 collar-cells, in this type of larva, are developed in the interior 

 before any flagellated cells have made their way there ; 

 and further, that they have been developed by fragmentation 

 of the nucleus and the subsequent division of the large cells 

 with vesicular nuclei found in type A, and have passed through 

 the condition found to be characteristic of type B. 



