500 E. A. MINCHIN. 



am.c.^). In some preparations of coriacea these minute 

 elements appear to be connected by a series of transitions with 

 the clear amoebocytes already described ; that is to say, the 

 latter by repeated division seem to break up into these exces- 

 sively minute cells. It is possible that this process represents 

 simply the method by which the wandering cells multiply. I 

 make this suggestion with all reserve, but am guided to it by 

 appearances, which I hope to describe in a future paper on the 

 embryonic development. For the present it is sufficient for 

 me to have described these cells simply for purposes of recog- 

 nition, in order to distinguish them from the cells of the 

 dermal layer proper. 



Apart from the sexual cells, of which it is not my intention 

 to treat in the present memoir, we can recognise two con- 

 stituent layers in the sponge body, besides a class of cell 

 elements which seem, properly speaking, to belong to neither 

 layer. These are — 



(1) The gastral layer, consisting of the collar-cells lining the 

 interior. 



(2) The dermal layer, consisting of — 



(i) The external neuro-muscular flat epithelium. 



(ii) The internal connective-tissue layer, consisting of the 



spicules and their formative cells, 

 (iii) The porocytes scattered about more or less evenly in 



the wall. 



(3) The aracfibocytes or amoeboid wandering cells met with 

 in all parts of the sponge. 



(c) System and Nomenclature. — In a former paper 

 (1896 [2]) I have put forward the outlines of the classification 

 which I intend to adopt. I recognised four genera of Ascons, 

 one of which has been seen as yet only by Haeckel. For these 

 genera I employed the names Clathrina, Gray; Leuco- 

 solenia, Bowerbank ; Ascandra, Haeckel; and Ascyssa, 

 Haeckel. 



The genus Clathrina is characterised by its reticulate 

 form, equiangular triradiate systems, collar-cells with basal 

 nuclei, })arenchymella larva, and " protascetta " stage in the 



