REPORT ON THE DEEP-SEA KERATOSA. 



13 



Histology. 



The tissues of the sponges are now generally regarded as derived from two simple 

 epithelial layers, which I first compared with the two primitive germ layers of the other 

 Metazoa, exoderm (or ectoderm) and entoderm (or endoderm), in my Monograph of the 

 Calcispongise (1872). From this comparison, and from the deduced homology of the 

 Gastrula form in all Metazoa, arose my Gastrsea theory. At that time I was of the 

 opinion that in all sponges these two primitive cell-layers were metamorphosed in a 

 similar manner, the inner (entoderm) lining as a simple permanent epithelium the 

 cavities of the gastral canal-system, and producing the sexual cells, whilst the blending 

 cells of the outer layer (exoderm) melt together and form a syncytium, or a contractile 

 protoplasmic ground-mass (sarcodine), in which the scattered nuclei of the cells are 

 propagating ; in this syncytium, too, the skeleton is formed. 



Three years later (in 1875) this conception was corrected by the accurate 

 observations of Franz Eilhard Schulze, the excellent spongiologist, who has advanced in 

 so many important directions the knowledge of this class of Coelenterata. Employing 

 new methods of histological examination, he discovered on the surface of many sponges 

 a delicate external pavement-epithelium not before observed, and deduced from this 

 observation the following important conclusions : — 



The body of the sponges is originally composed not of two, but of three primitive 

 cell-layers, corresponding to those which in the higher organised Metazoa are called 

 exoderm, mesoderm, and entoderm. The exoderm (or outer layer covering the external 

 faces) and the entoderm (or inner layer lining the canal-system internally) are two 

 simple epithelial plates, and between them is enclosed the mesoderm (or the middle 

 layer) ; this latter is a kind of connective tissue, and produces not only the skeleton, 

 but also the sexual cells (eggs and sperm). 



The conception of the sponge-tissues given by F. E. Schulze is now generally 

 accepted, and it is very probable that it has general value, though it was not possible to 

 demonstrate clearly in all sponges the delicate exodermal epithelium. The histological 

 examination of our Deep-sea Keratosa has given no remarkable and positive results in 

 this respect, owing to their insufficient state of preservation. I will not, therefore, 

 further discuss their finer histological structure, but only add a few remarks on the three 

 above-mentioned layers. 



Exoderm (Surface-Epithelium). — The delicate simple epithelium, composed of thin 

 pavement-cells, which F. E. Schulze discovered on the surface of many sponges, is now 

 usually regarded as an independent cell-layer, and often compared with the epidermis of 

 the higher Metazoa. This conception may be combated even when we assume its 

 general presence in all sponges (which is not proved). In my opinion this outer 

 exodermal epithelium does not possess the same primary importance and independence as 



