410 M, K. A. Zittel oa Fossil HexactinelUda. 



theless the arrangement of the subjacent latticed framework 

 may still be recognized in the covering layer from the regular 

 position of the axial canals. (Examples, Marshallia^ Callo- 

 dictyon^ Pleurope, Plocoscyjjhia, under surface of Coelopti/chium, 

 Craticidaria, &c.) 



Sometimes covering layers are produced by the uppermost 

 layer of sexradiates of the skeleton emitting root-like processes, 

 Avhich anastomose with each other, and in this way form a 

 felted tissue consisting of solid siliceous fibres. This either 

 coats the surface of certain parts of the sponge-body, or it only 

 fills the meshes of the uppermost layer of the latticed tissue 

 [Etheridgia) . 



The covering layer attains a greater amount of indepen- 

 dence of the true skeleton when it forms a coarsely or finely 

 perforated siliceous membrane in which axial crosses of the 

 size and form of those in the skeleton-spicules lie irregularly 

 dispersed. [Sporado2)yle, Graticularia^ SjjhenaulaXj Sjoora- 

 doscimon^ &c.). 



Independent covering layers originate in quite a difierent 

 manner when the outer, the inner, or both surfaces of the 

 sponge-body are veiled by a coat of sexradiates, resembling 

 sometimes an extremely delicate spider's web, the sexradiates 

 being variously cemented together either only by their points 

 or by the whole length of the limb ( Casearia^ Cypellia, Tre- 

 viadictyon, &c.). In these sexradiates, which sometimes differ 

 very essentially from those of the rest of the skeleton, the 

 outwardly directed ray is also always aborted. In certain 

 genera [Cypellia) the large sexradiates of the covering layer 

 are sometimes only united at their tips by slight siliceous 

 bridges ; in others they lie like stars imbedded in a thin sili- 

 ceous membrane which coats the surface of the sponge {Foro- 

 cypellia, Porospongia) . 



In those cases in which the spicules of the covering layer 

 are of a different form from the skeleton-spicules, they no 

 doubt take the place of the flesh-spicules ; but whether this is 

 the case when the covering layer is produced only by a thick- 

 ening of the skeleton itself, appears to be doubtful. The 

 question can only be settled when we know living Hexacti- 

 nelUda with the surface of a similar constitution. 



In some fossil Dictyonina [Cystispongia, Lepidospongia) the 

 outer surface of the sponge-body is coated with a dense sili- 

 ceous membrane, which is entirely free from axial crosses or 

 enclosed spicules. 



These variously developed superficial structures evidently 

 are intended to give the sponge-body greater firmness, and to 

 protect it against assaults from without. The covering layers 



