12 THE VOYAGE OF H.M.S. CHALLENGER. 



Suborder I. DICTYONINA, Zittel. 



Skeletal spicules fused in such a way that every arm of a six-rayed spicule is applied 

 to the corresponding arm of a neighbouring spicule, so that both become surrounded 

 by a common siliceous covering. The continuous skeleton consists of a framework, 

 with cubical or irregular meshes. Flesh spicules present or absent. 



Family 1. Astylospongid^, Zittel. 



Sponge body very thick walled, unstalked, free (occasionally fixed by a broadly 

 expanded base). Water vascular system consisting of radial canals extending from the 

 surface to the centre, besides which vertical tubes disposed in radial rows to the number 

 of eight or ten, are generally present. Lattice framework tolerably irregular with 

 thick nodes of intersection. 



Only fossil forms from the Silurian. 



Family 2. Euretid^. 



Fixed sponge bodies beaker-like, cylindrical, top -like or branched. Skeleton lattice- 

 like ; the intersection nodes of the fused hexradiate spicules non-perforated. Surface 

 naked or protected by a thickening of the outer skeletal layer, sometimes covered with 

 a very delicate network of fused spicules, which, in theii* form, differ but slightly from 

 those of the rest of the skeleton. This outer dermal meshwork also surrounds the 

 ostia. Structure of the root resembling that of the rest of the sponge body. Flesh 

 spicules absent or present. 



(a) Canal system well developed. Ostia of the blind radial canals occur variably on 

 either surface. In addition to fossil genera belonging to different strata, this suljfamily 

 is represented by the living genus Sclerothamniis, Marshall. 



(b) Canal system absent or scarcely developed. Besides the fossil genus Verruco- 

 ccelia, the living genera Farrea, Bowerbank, Eu7-ete, Marshall, and Aulodictyon, S. Kent. 



Family 3. Coscinopori 'dm, Zittel. 



Sponge body beaker-like, stellate or branched, more frequently compressed. Eadial 

 canals very numerous, simple, straight, and blind. Ostia small. Skeleton finely meshed, 

 dense, stony. The numerous radial canals exhibit the regular formation of cubical 

 meshes. The intersection nodes of the six-rayed spicules are thick and seldom per- 

 forated. Dermal layer usually absent or only formed hj a thickening of the outermost 

 skeletal layer. 



Examples. — Coscinospora and other fossil genera. 



