54 COELENTERATA SPONGIAE CLASS i 



Amphithelion, Zitt. Like the preceding, but with both ostia and postica 

 terminating in bosses. Cretaceous. 



Other genera : Scytalia, Coelocorypha, Stachyspongia, Pachinion, Seliscothon, Zitt. 

 etc., in the Middle and Upper Cretaceous. 



Order 4. HEXACTINELLIDA. O. Schmidt. 

 (Triaxonia, F. E. Schulze.) 



Silicious sponges with six-rayed skeletal elements, the rays being normally disposed 

 in three axes intersecting at right angles, and containing axial canals ; elements either 

 detached or fused together so as to form a lattice-like mesh. Dermal and flesh spicules 

 exceedingly variable in form, but invariably six-rayed. 



Next to the Lithistida, the Hexactinellida are the most abundant of the fossil 

 silicious sponges. They are extraordinarily variable in form, and are often 

 anchored by a tuft or " rope " of long, slender, glassy fibres, or are attached 

 directly by the base. The walls are thin, as a rule, and enclose usually a wide 

 cloaca ; the canal-system is consequently much simpler than in the Lithistida, 

 being made up merely of short tubes which penetrate the walls more or less 

 deeply on both sides, and generally end blindly. Sometimes the sponge is 

 entirely composed of thin-walled tubes which twine about one another irregularly 

 and produce a system of lacunar interstices (intercanals) of greater or lesser size. 



The skeletal elements proper are distinguished by their considerably larger 

 size and uniform type of structure, from the usually minute, astonishingly 

 variable and delicate flesh-spicules ; the latter, unfortunately, are very seldom 

 preserved in the fossil state. The skeletal elements occur detached in the soft 

 parts in the Lyssacina group, or they are but partially or irregularly cemented 

 together ; in the Dictyonina group, on the other hand, the skeletal elements are 

 regularly united in such manner that the rays of proximate elements are all 

 closely applied against one another, and are surrounded by a continuous silicious 

 envelope. In this way a more or less symmetrical lattice-work with cubical 

 meshes is produced, in which, however, the fusion of juxtaposed elements is 

 indicated by the fact that each ray contains two distinctly separated axial 

 canals. The junction of the rays at the central node of each element is usually 

 inflated, but is .sometimes sculptured in such manner as to enclose a hollow 

 octahedron (lantern nodes, lychnisks). The exterior of the skeleton is often 

 covered by a dermal' layer composed of irregular hexactins, in which the externally 

 directed ray has become atrophied ; or a dense silicious envelope is secreted, in 

 which stellate hexactins with reduced outwardly and inwardly directed rays 

 (stauractins) are embedded in greater or lesser profusion. 



The Hexactinellida of the present day are distributed chiefly over the greater 

 depths of the ocean beyond the hundred fathom line (200 to 3000 fathoms). 

 They occur fossil principally in deep-sea deposits, and make their first appear- 

 ance in the Cambrian ; their period of greatest development falls in Jurassic 

 and Cretaceous time. 



Sub-Order A. LYSSACINA. Zittel. 



Skeletal elements either entirely detached, or only partial! // and in an irregular 

 fashion cemented together. Root-tuft often present. 



