412 M. K. A. Zittel on Fossil HexactinelUda. 



when, tlie wall of the sponge-body being thin, the latticed 

 skeleton and the surface are of such coarse meshes that the 

 water can pass in and out unhindered. In this case true 

 canals are superfluous, and in the living state each mesh-, 

 opening of the surface of the skeleton probably corresponded to 

 a dermal pore in the syncytium. Examples of Hexactinellida 

 with a coarsely-meshed skeleton without any canals or ostia 

 are presented by the genera Farrea, Eurete^ Myliusia^ Mar- 

 shallia, and CaUodictyon. 



In certain Hexactinellida the water-circulation may be 

 effected, in the absence of true radial canals, by a strong fold- 

 ing of the wall. For example, there are Ventriculites in 

 which the wall lies in mgeandriform folds, between which, on 

 both surfaces, deep longitudinal furrows run from the upper 

 margin of the cup down to the base. These furrows quite 

 suffice for the permeation of the sponge-body; and conse- 

 quently all radial canals or large superficial apertures may be 

 deficient. 



But if in Hexactinellida with maeandi-iform -folded wall the 

 folds lie close together, and in this way constitute a continuous 

 wall {Vent7-icidites) , or if the wall attains an extraordinary 

 thickness {Pachyteichisma) , the necessity for a canal-system 

 capable of conveying the water into the interior of the skeleton 

 is established. Therefore the cgecal radial canals, the ostia of 

 which are variously distributed on the two surfaces, then make 

 their appearance. 



In general it may be recognized as the rule in the Hexacti- 

 nellida, that the development of the canal-system keeps pace 

 with the tliickness of the wall, or Avith the density of the 

 latticed framework. All genera with a very finely !rneshed 

 tissue possess canals, even when the wall is reduced to a 

 lamella no thicker than paper [Leptophragma Murchisoni). 



A very peculiar and complicated canal-system is possessed 

 by some of the most ancient Hexactinellida, with globular, 

 stalkless sponge-bodies (Astylospongidce). In these remark- 

 able sponges the wall is of very considerable thickness ; the 

 central cavity is either entirely deficient or it occurs as a 

 funnel-shaped depression on the upper surface. The whole 

 mass of the sponge-body is traversed by numerous simple 

 canals arranged in radii, running from the periphery towards 

 the centre. These radial canals are crossed by other stronger 

 canals, which are likewise arranged in radial series, but 

 traverse the sponge-body through its whole height, running 

 parallel to tlie sui'face. This constitution of the canals, which 

 never occurs in living or mesolithic Hexactinellida, almost 

 exactly represents the water-canal system observed in some 



