386 RICHARD EVANS. 



to the chamber in an adult sponge. In reality the communi- 

 cation between these two kinds of spaces is an already deve- 

 loped exhalant pore or apopyle. It may further be pointed 

 out that these spaces, whether canals or chambers, are struc- 

 tures formed in situ from the cells of the solid pos- 

 terior end of the inner mass, and not as ingrowths 

 into it from the layer of cells which line the larval 

 cavity. Should some of the short canals appear to open into 

 the larval cavity, the communication is one which has been 

 secondarily acquired during the development. 



The cells with vesicular nuclei are less numerous, more 

 irregular in shape and outline, and smaller in size than they 

 are in types A and B. They become less numerous owing, 

 on the one hand, to their conversion into cells with granular 

 nuclei, and, on the other hand, to the formation out of them 

 of the cell groups. They become more irregular in shape 

 owing apparently to the stored-up food material being used up, 

 giving them greater facility for change of form and the 

 exercise of their wandering function. They become smaller 

 in size owing to repeated cell division. In this type it is 

 quite exceptional to find a cell measuring more than 10 fi 

 across, while in type A. they often measure from 12 to 15 fi, 

 and in type B from 10 to 13 ;u. A nucleus measuring 7 /x 

 across is often seen in types A and B, but in type C the 

 largest vesicular nucleus seldom exceeds 5^ ju. in diameter. 

 On the other hand, a vesicular nucleus measuring less than 5 fx 

 across is scarcely found in types A and B, but in type C tiiey 

 constantly occur. The same reduction of size is noticeable in 

 the central corpuscle (or nucleolus) of the vesicular nucleus. 

 In types A and B it often measures 2 fx across, but in the type 

 now under consideration it averages only 1^ n in diameter. In 

 ty[)es A and B these cells contain one or more nutritive 

 vacuoles and several yolk bodies, but in type C they seldom 

 contain more than one of the former, which have also become 

 slightly reduced in size. The number of the yolk bodies is 

 also greatly decreased, though they are by no means completely 

 absent. Moreover, the cells in question often produce spicules 



