392 Mr. H. J. Carter on the 



the cavity in the embryo of Orantia compressa lias the same 

 appearance as that about the bunch of root-cells at the base of 

 the embryo of Halichondria simulans, the root-cells of the 

 embryo of Grantia compressa first make their appearance in 

 this part of the embryo, and hence have been called by Hackel 

 the " endodermal layer " — erroneously, I think, because the real 

 endodermal mass or layer appears to consist of the sarcode 

 charged with cells and granules which I have stated to exist 

 between the brown- coloured cavity and the ectodermal layer 

 (see also Lieberkrihn's figure for this, I. c). 



Whether the root-cells in the embryo of Halichondria 

 simulans make their first appearance in the centre of this body 

 I cannot pretend to say, on account of its opaqueness at 

 maturity and during its previous stages ; but that they do so 

 in the embryo of Halisarca lobularis its transparency enables 

 one to see distinctly, as our illustration will show (PI. XX. 

 fig. 12, erf). 



Fourth Period. 



On account of the microscopic size of the embryo of Grantia 

 compressawherx it leaves the parent (PI. XX. fig. 13), compared 

 with that of Halichondria simulans (PI. XXI. fig. 21), which 

 may be seen with the unassisted eye, I could not treat the former 

 in the same way as the latter to follow its development through 

 the jirst part of this period, and therefore was obliged to have 

 recourse to the examination of objects in the residuum men- 

 tioned, which might perchance afford these stages. This was 

 not difficult, so far as the young Grantia compressa when fully 

 developed was concerned, as there were many specimens of it 

 in the residuum as well as on the dried pieces of Ptilota bear- 

 ing the young and old forms together, which, only being a 

 trirle larger than the embryos in their bodies respectively, as 

 may be seen by the illustrations (PI. XX. figs. 13 and 18), 

 might be assumed to be the earliest form of the perfected 

 sponge. 



I, however, was so fortunate as to meet with two specimens 

 where the respective stages between the fixation of the em- 

 bryo and the fully developed sponge were amply represented ; 

 in one of which (fig. 16), the dermal membrane (a) still re- 

 tained its even, round continuity, while the interior only con- 

 tained four triradiate spicules (fig. 16, b) in the midst of a mass 

 consisting of sarcode charged with cells of a uniform size, viz. 

 about 2 6000ths inch in diameter, and granules (fig. 16, c), 

 but with no appearance now of the larger " root-cells." 



In the other specimen (fig. 17) the triradiate spicules were 

 more numerous (fig. 17 b). and there were several of the cha- 



