52 BRITISH FOSSIL SPONGES. 
delicate vertical laminz, 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 Corallidiwnm 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 
