82 BRITISH FOSSIL SPONGES. 
spicules (Fig. 4, )). As these facets are very numerous, and radiate in all direc- 
tions from the main axis of the spicules, they form by their union an extremely 
intricate meshwork with minute irregular interspaces. In some instances the 
spicules thus united produce anastomosing fibres with open interspaces, in which 
the circulation is carried on, as, for example, in Pachinion, Zitt.; in others, as in 
Seliscothon, Zitt., they form thin vertical lamelle. 
The spicules of the dermal layer in lithistid Sponges are arranged so that the 
small vertical ray penetrates into the wall of the Sponge, whilst the horizontally 
extended head-rays cover the outer surface, frequently overlapping each other, 
thus forming a smooth covering with only microscopic interspaces. They are not 
organically attached to each other, or to the skeletal spicules, and it is very 
seldom that they have been preserved in situ in the fossil state. In some cases 
the dermal spicules appear to have extended over the cloacal surface as well as 
over the exterior of the Sponge. In the genus Doryderma, the trifid spicules of the 
dermal layer are tightly packed, with their shafts parallel to each other, into the open 
meshes of the skeletal-spicules, and their head-rays project, like arrows in a quiver. 
Hevactinellidg.—In this division the six-rayed spicules are disposed so that 
their rays overlap each other, and produce a framework which, when regularly 
developed, has quadrate or subquadrate boundaries. The spicules may be simply 
held in position by the sarcode or fleshy portion of the Sponge, or they are united 
organically together by a common siliceous sheath. Fossil examples of those 
Sponges in which the spicules are not organically fused together are very rare, 
though the detached spicules are extremely abundant. Judging by existing forms 
of these Lyssakine Sponges, the spicules are generally arranged into elongate, 
loose fasciculate fibres or open tissues, which in some genera, as in Huplectella, for 
example, cross each other in regular lines. Not strictly in all cases are the 
spicules free from each other, for they are sometimes soldered together laterally, 
and at the crossing of some of the larger rays, by a deposit of silica. In many 
recent Lyssakine Sponges the spicules of the dermal layer have the same regular 
arrangement as those of Dictyonine Sponges, and form a meshwork with quadrate 
interspaces, but the spicules are not fused together. A fragmentary example of 
this is shown in Hyalostelia Smithii (Pl. VI, fig. 1). The elongated spicules of 
the anchoring rope of this group of Sponges are disposed either parallel to each 
other, so as to form rounded or compressed fascicles (Pl. I, fig. 3; Pl. VI, figs. 2, 
3d), or they extend singly through the rock. Though the component spicules of 
these ropes are not organically attached together, yet they are found in close 
contact with each other for considerable distances, and this undisturbed arrange- 
ment may probably be owing to the circumstance that these anchoring ropes 
were embedded in the mud during the life of the Sponge, and were thus preserved 
from disintegration after its death. 
