222 ARTHUR DENDY. 



In Leucandra phillipensis the ordinary amoeboid cells 

 are very much larger than the cells in question in the same 

 sponge; in fact, compared to the latter and to the collared 

 cells they are as giants. Curiously enough^ I observed one of 

 these large amoeboid cells, shown in fig. 50, apparently feeding, 

 by means of pseudopodia, on the collared cells. The chamber 

 to which the collared cells belonged was, however, in the ordi- 

 nary uncontracted condition, and I could not see any indication 

 of the division of the collared cells into '' column and plinth.^^ 



On the whole, the peculiar contracted condition, such as is 

 shown in fig. 24, and which occurs in chambers both of the 

 Syconoid and Leuconoid types, is perhaps to be regarded as 

 indicating not so much a process of death as one of re- 

 juvenescence, and this is, after all, but a slight modification of 

 my former view. We may suppose that, after perhaps taking 

 in a large supply of food, the chamber passes into a resting 

 condition. The collared cells then undergo certain changes 

 which are not fully understood, but which finally result in 

 the formation of a new chamber with ordinary active collared 

 cells. The contraction of the chamber during this process 

 appears to me to be normal, and not due to the action of re- 

 agents. 



The Mesoderm. 



The transparent gelatinous ground substance which forms 

 the bulk of the mesoderm in all calcareous sponges, appears to 

 vary only in the extent to which it is present and the propor- 

 tion which it bears to other parts of the sponge. It is most 

 strongly developed in the dermal and gastral cortex (figs. 10 

 — 12). It may also be fairly abundant in the walls of the 

 flagellated chambers, as in Sycon gelatinosum (figs. 3 — 5), 

 but it is usually very sparingly developed in this situation, so 

 as to be distinctly recognisable only at the points where the 

 chambers touch one another (fig. 13) . Hence in very many cases 

 the walls of the chambers appear in thin sections to be made 

 up of two contiguous layers of cells, the collared cells on the 

 inside and the pavement epithelium of the inhalant canals on 



