20 



Dun'YOSPONGID^. 



mi iMi 



f an HI! 



iitl /ill 



mi iiHj 



III! !■■«' 



I nil la 



fill ml 



nil iiii 



ill I 

 •"J L- 



••■1 Hlli 



  !!!! 

 1 11 "SL 

 nil i»t 



nil " 



IIII 



Mil 



 Ini mi 

  iiii ill, 

 !. ill 



The fossil species show that these fascicles of rod-like spicules lie upon the 

 l)aragastnil sui-face and a given spicule is often continuous from one extremity 

 of the sponge to the other. It seems evident that these spicules have no 

 connection with the basalia and marginalia, though the latter are continued 

 into the substance of the sponge and may extend through to the paragastral 

 surface. In the living Euplectella, the ground work of the skeleton is of 

 similar chanicter, and students are generally agreed in regarding the cylindrical 

 rods composing the horizontal and vertical bundles as modifications of 

 cruciform spicules, due to extreme prolongation of the rays of one axis and 

 the complete suppression of the others. In the Dictyosponges these fascicles 

 sometimes include rods terminating in anchors, and others with a series of 



barbs along the sides (denies) ; a fact which 

 may tend to corroborate the derivation above 

 assumed. Under this interpretation these 

 rod-like 8j)icules are of different nature from 

 the prostalia ; as they are evidently gastral 

 in position they are properly termed gaMra- 

 lid and may be distinguished from less modi- 

 fied gastral spicules lying in the interspaces 

 of the network by the term gastralia later- 

 alia. Such rod-shaped spicules when clearly 

 preserved in pyrite show very distinctly a 

 tubular axial cavity about which the sub- 

 stance of the rod is arranged in concentric 

 bands or envelopes. Other spicular elements 

 of the Dictyosponges such as have been 

 clearly made out, can be referred to the outer 

 surface of the cup {dermalid) and there are 

 a few which it may be safe to regard as rep- 

 resenting the medial layer or pai'euchyma of 

 the thick-walled sponges (parenclnjinalia). 

 The variations in the spicular composition of 

 the skeleton will be considered at greater 

 length in following pages. 

 Taxonomy. The siliceous sponges constituting the Class SILICEA, 

 (Gkay, 1867), or those whose skeleton is essentially composed of siliceous 

 spicules*, are divided into three orders, based iipon the form of the 



•Recent writers (cf. Raupp, op. oil.) iacliide in tho Silicea, the ceratose or horny sponges. 



n 



__ *' •■■■ 



Hrii >!■■; nil 



s,'masiiai^ 



< nil ml I 



\\ii ml  nil iis; 

 \ii* •••■  »n '■-' 



uii nil  IHiUSS 



ull nil  III! tmtm , 



Figure 8. Diagrammatic representation of the 

 arrangameDt of the spicular parts iu a typical Dlctyo- 

 sponge. 



