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-endoderm of orthodox scliematic morphology, does not form the inner- 

 most layer of the body-wall, but is suspended in the middle of the dermal 

 layer, which thus forms both the inner and outer layers of the sponge- 

 body. Nevertheless, I may point out, this fact is by no means without 

 a parallel in sponges belonging to other groups. In the embryonic 

 development of Ascons I have shown [7 p. 71] that the gastral cavity 

 at its first appearance is entirely surrounded by cells of the dermal 

 layer, which form at this early stage a continuous epithelium shutting 

 out the cells of the gastral layer, the future collar-cells, from contact 

 with the gastral cavity. At a later stage these internally situated dermal 

 cells migrate outwards to form the pore-cells of the Ascon, a class of 

 cells apparently not represented in the Hexactinellids ; but even in the 

 adult Ascon, the pore-cells return to their primitive position on the 

 interior of the collar-cell layer whenever the sjionge passes into the 

 condition of complete contraction, and pass out again as the sponge 

 becomes once more expanded. Further, in my account of Clathrina cori- 

 acea [6], I showed that in one form of the sponge a network of trabeculae, 

 formed of cells of the dermal layer similar to the pore-cells, is always 

 found extending through the entire gastral cavity. Similar endogastral 

 networks have been described by Dendy [1, p. 13. pi. 8. fig. 1, 2] in 

 other Calcarea, and appear to have been seen also by Haeckel [2, III, 

 pi. 22, fig. 3c; pi. 58, fig. 4 etc.]. Such networks are comparable in 

 every way to the trabecular system of Hexactinellid sponges, but though 

 the facts were known to me their significance was not clear to my mind 

 until Ijima published his investigations upon Hexactinellids. The inter- 

 pretation which I place upon them is that the gastral layer, the collared 

 epithelium, is not the primitively innermost layer of the sponge-body, 

 but was originally suspended in the midst of the dermal layer, as we 

 still find in Hexactinellids. In Calcarea, Avith the development of the 

 supporting spicular skeleton, the whole of the dermal layer has come to 

 place itself to the exterior of the gastral layer, but the primitive state of 

 things is still found not only in the embryo, but even in the adult when 

 the conditions of the environment make the additional support of an 

 endogastral network advantageous to the sponge. In the Demospongiae 

 there is less evidence, so far as I am aware, for the former existence of 

 an internally placed portion of the dermal layer, but it has seemed to me 

 possible that the much-discussed central cell of the flagellated chamber, 

 first observed by Delage in some Monaxonida, might represent a last 

 remnant of dermal cells situated internally to the collar-cell layer. I may 

 finally point out that the gastral layer being primitively not the most 

 internal layer of the body-wall, is a hypothesis which makes the reversal 

 of the layers at the metamorphosis much more intelligible , and bridges 



