52 BRITISH FOSSIL SPONGES. 



delicate vertical laminse, with narrow, simple, or branched intervening fissures so as 

 to give the Sponge a radiate appearance like that of a Coral. The canals follow 

 the course of these fissures, thus running radially from the outer surface in a 

 generally horizontal direction to the centre of the Sponge. Their apertures are 

 disposed in the fissures over the lateral surface in vertical rows. The vents are 

 situated on the upper or inner surface of the sponge-wall. In this division are 

 included the Jurassic genera Cnemidiastrum and Gorallidium and the Cretaceous 

 Seliscothon. 



Canal-system of Fossil Hexactinellids. — As the mode of growth in the majority of 

 fossil hexactinellids consists of a thin wall of spicular meshwork, offering a largely 

 extended free surface to the surrounding medium, the canal-system is not of so 

 complex a character as in lithistid Sponges, in which the skeleton is much thicker. 

 In general there is a system of short, blind canals penetrating the thin wall on 

 both sides at right angles to the surface, and extending nearly through it. In 

 addition to these, in some cases a system of pseudo-canals or inter-canals is pro- 

 duced, by various infoldings in the walls themselves, and by the development of a 

 supplementary dermal layer of spicular tissue lining the outer surface of the 

 Sponge and the cloacal cavity, and thus forming a sac-like cavity in which the 

 folds of the wall are contained. The following are some of the modifications of 

 the canal-system in this group. 



(1) In which special canals are not present, or are not indicated in the spicular 

 meshwork of the skeleton. In this case the sponge-wall consists of a simple ex- 

 tended layer of spicular mesh, in which only the ordinary quadrate interspaces 

 between the spicules are present. An outer modified spicular dermal layer may be 

 present with regularly disposed apertures, but these are not connected with canals. 

 The genus Callodictyon is a typical example. 



(2) In which there are special canals extending quite through the sponge-wall. 

 In general appearance the wall of the Sponge differs but little from that of the 

 preceding, but on close examination it is seen that the tubular canals which pene- 

 trate quite through it, at right angles to the surface, are not mere ordinary 

 openings in the spicular mesh, but they are definitely bounded by a modified 

 arrangement of the spicular rays of the mesh. A dermal layer is present. This 

 modification is clearly shown in the genus Aphrocallistes. 



(3) In which the canals terminate blindly in the sponge-wall. In this case the 

 small tubular canals are present on both surfaces of the sponge-wall, and communi- 

 cate with the exterior by small circular apertures. They reach nearly through to the 

 opposite surface of the wall and terminate blindly. They are usually disposed in 

 alternate rows on the respective surfaces, so that in the interior of the wall the canals 

 are side by side. It seems probable that these blind canals belong to the excurrent 

 system and that the water entering through the smaller apertures of the dermal 



