STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 1833 
s 
is very variable, according to the number of spicules entering 
into their composition at any given point. They are seldom 
more than about 0:07 mm. in diameter, except towards the 
dermal surface, where their thickness may be greatly aug- 
mented by the addition of numerous slender oxeote spicules as 
already mentioned. Towards the gastral surface the fibres 
often appear, as it were, to spring from the long basal rays of 
the subgastral sagittal triradiates (figs. 4, 5), a relation which 
is of considerable importance in considering the derivation of 
this peculiar type of skeleton from the primitive articulate 
type. Except just close to the two surfaces of the sponge-wall 
the fibres appear to consist solely of the remarkable tuning- 
fork-shaped triradiates. All the spicules of any one fibre, so 
far as I have been able to make out, have their basal rays 
pointing in the same direction. Usually the fibres have a 
distinct, though more or less oblique trend from gastral to 
dermal surface, and it is extremely interesting to note that the 
basal rays of the component spicules iu such cases almost 
always point towards the outside of the sponge, a fact which 
has already been noted by Mr. Carter. 
(c) The huge Oxeote Spicules. These occur in im- 
mense numbers, disposed in the utmost confusion between the 
spicular fibres (fig. 3). The thickness of each one is greater 
than that of an average fibre, and, indeed, the fibrous portion 
of the skeleton can only play a part of secondary importance 
in strengthening the sponge-wall as compared with these giant 
spicules. 
Skeleton of the Peristome.—This appears, from Mr. 
Carter’s description, to present no very characteristic or i1m- 
portant features. It consists of long, straight, slender oxeote 
spicules arranged perpendicularly (parallel to the long axis of 
the sponge). The lower ends of these spicules are crossed at 
right angles and supported by the outspread lateral rays of the 
gastral triradiates. 
