Development of the Marine Sponges. 397 



think, be regarded as a temporary production of rooting cells 

 (PL XX. figs. 13 & 15, d). I allude to the bunch of large cells 

 at the posterior extremity, which originally comes from the 

 centre of the embryo (see that of Halisarca lobularis, PI. XX. 

 fig. 12, d) 7 and has, as before stated, the real ectoderm (in 

 which the spicules and skeleton-structure, together with the 

 spongozoa and ampullaceous sacs, are developed) between it 

 and the ectodermal layer. 



That these naked plastic cells should be engaged in rooting 

 the embryo, and not the monociliated cells of the ectodermal 

 layer, which has for its part at the commencement the loco- 

 motion alone of the embryo, seems to me to be by far the most 

 probable conclusion, even if we had not the fact analogically 

 demonstrated by the development of the embryo in Hali- 

 chondria simulans. 



As regards the cavity presented by the purse or bottle-like 

 forms of the calcareous sponges, ex. gr. Grantia compressa, 

 Grantia ciliata, &c, called by Dr. Bowerbank the " cloaca^" 

 into which the excretory canals empty themselves, this is 

 nothing more than the dilated extremity of the excretory canal- 

 system modified, and as common in the tubular or hollow digital 

 forms of the siliceous sponges as in the calcareous ones ; while 

 the more common form, in which the excretory canal-system 

 is accompanied by no such cloacal termination, renders this 

 structure in the calcareous sponge Leuconia nivea &c, identi- 

 cal with that of the sessile spreading form of Halichondria 

 simulans. 



Thus, when we consider the resemblance in form that exists 

 between the embryo of the calcareous and that of the siliceous 

 sponges, ex. gr. Halichondria simulans, and that the papilla 

 at the end of the latter becomes the vent of its excretory canal- 

 system, which in form is identical with the excretory canal- 

 system of the calcareous sponge Leuconia nivea, and, but for 

 its presenting the purse-like tubular modification in Grantia 

 compressa, is in the latter equally identical, it is difficult to 

 conceive that in the embryo of the calcareous sponges this 

 system is developed in any way different from that in the 

 embryo of the siliceous sponges. 



How the excretory canal-system is produced I am not pre- 

 pared to say. It may be by an inversion or extension inwards 

 of the ectodermal layer. But whatever this may be, to reverse 

 the embryo in the calcareous sponges for this purpose, as done 

 by Hackel, seems to me to be utterly unsupported. I have 

 already stated that the microscopic minuteness of the embryo 

 of the calcareous sponges, when it issues from the parent, pre- 

 cludes the possibility of following it to its place of settlement 



