680 \l. KIRKPATUICK. 



maltha as to resemble an epithelium wheu viewed from the 

 surface. In some very thick vertical sections, well preserved 

 aod stained, it was possible to see a considerable area of the 

 surface as well as the depth of the sponge, and to note the 

 entire absence of any trace of an epithelium. Accordingly, 

 Merlia in its adult condition affords a conKrmation of the 

 theory put forward by Haeckel and confirmed by the embryo- 

 lou'ical researches of Ma;is, Yves Delate and Miucliin, that 

 the sponge is a two-layered oi-ganism. Merlia has an " exo- 

 derin " and an " endoderm," but no mesoderm. 



Although there is no epithelium on the surface of the 

 sponge the canals have an epithelial lining, which apparently 

 consists of cells of the same nature as the colleucytes in the 

 maltha, for sometimes a cell on the surface of a canal has 

 branching processes extending back into the maltha. 



(d) The Myocytes. 



The myocytes form concentric or radial groups round the 

 incurrent and excurrent orifices. The myocytes have a 

 granular plasma, the granules being coarser than those of the 

 collencytes, and the nucleus is vesicular and without a nucle- 

 olus. They vary greatly in shape, but generally have branched 

 prolongations — indeed, they do not differ greatly from the 

 collencytes. Sometimes the branched prolongations surround 

 an orifice and fuse, so that the myocyte becomes a poroeyte 

 (PI. 37, fig. 3). Further, one or more small, curved, fusiform 

 myocytes surround the apopyle of each flagellated chand)er. 



(e) Scleroblasts. 



The scleroblasts are very finely granular, and with a clear 

 splierical nucleus. PI. 36, fig. 2 shows these cells both in 

 longitudinal and transverse sections of spicule bundles. PI. 

 36, fig. 10 shows a scleroblast of one of the clavidiscs. 



