M. K. A, Zittel on Fossil HexactinelUda. 413 



Lithistida. In the Astylospongidai, however, the canals 

 appear to have exercised no influence upon the microstructure 

 of the skeleton-spicules ; for these are as sharply discriminated 

 from the skeleton of the Lithistida as those of the later 

 HexactinelUda. 



Besides the true water- vascular system there is in some 

 HexactinelUda also a so-called " intercanal system." The 

 tubular cavities and larger or smaller apertures on the surface 

 to which Hackel* gives this name are produced simply by 

 the peculiar coalescence of certain parts of the sponge-body. 

 Physiologically they have nothing in common with the true 

 canal-system ; and they never show any constant characters in 

 their course or their form. What Hackel says of the inter- 

 canal system of the Calcispongia applies also essentiallv to 

 the HexactinelUda ; and on this subject I may therefore simply 

 refer to the classical monograph of the Calcispongise. 



In the HexactinelUda an intercanal system occurs only on 

 composite stocks, and, indeed, chiefly when the cormi are 

 composed of tubes which grow labyrinthically through one 

 another and leave irregular interspaces free. The intercanal 

 system is developed in a remarkable manner in Etheridgia, 

 Tremaholites.) Cystispongia, and Plocoscyphia. 



We must be very careful not to confound the apparent 

 stomachal cavities (pseudogastres) and orifices (pseudosto- 

 mata) formed at the surface by the intercanal system {Ethe- 

 ridgia, Plocoscyphia, &c.) with true gastral spaces and their 

 apertures. An investigation of the depression will in most 

 cases at once show that such apparent stomachs are not im- 

 mersed in the true skeletal mass and bounded by a continuous 

 wall, but that they rather represent interspaces of usually 

 irregular form, the walls of which are formed by the outer 

 surface of parts of a sponge-body of different values. 



3. Although the external form of the sponge-body is gene- 

 rally subject to the greatest variations and is scarcely taken 

 into consideration in all the more modern systems of living 

 sponges, it nevertheless merits a higher estimation in the case 

 of the vitreous and calcareous sponges with firm, stony skele- 

 tons. It is true that we can only exceptionally recognize 

 genera at once from their characteristic external form [Coelo- 

 ptychium, Euplectella) , as, in general, the same forms are 

 exactly repeated in the HexactinelUda, the Lithistida, and the 

 Calcisponglai with anastomosing fibres. Moreover it is abso- 

 lutely impossible to determine generically a cup-shaped, bowl- 

 shaped, funnel-shaped, or cylindrical sponge of the order 



♦ Die Kalkschwamme, i. p. 275. 



