708 
DE. WTVILLE THOMSON ON HOLTENIA. 
spicules, while their tree-like branches project outwards, thinly covered with transparent 
sarcode. Frequently one of these spicules is carried away by the stream of sarcode 
spreading into the ultimate sarcode-net, and may be seen upon the netting, maintaining 
its position, with the cross in the network and the brush projecting outwards (Plate 
LXIX. fig. 3). 
The small amphidisci (Plate LXVIII. figs. 15, 16) are less numerous. The position of 
these bodies in the sarcode appears to be perfectly irregular ; they lie in all directions. 
They are usually found ranged along the rays of the large spicules ; but they also often 
follow the course of the sarcode, and may be seen scattered in the threads of the ulti- 
mate network (Plate LXIX. fig. 3). 
When the sponge is fresh, with only the mud washed off it, if we look at it in water 
the spaces between the larger stars seem almost transparent, and show deep cavities 
penetrating into the substance of the sponge, while beneath the centre of each star an 
irregular column of sponge-substance rises, spreading somewhat and dividing as it abuts 
against and supports the star. These supporting columns have a great resemblance in 
form to the columnce carnece of the human heart. 
Over the whole of the lower third of the surface of the sponge, dense tufts, consisting 
each of from 20 to 100 of the enormously long simple spicules, portions of which are 
represented in fig. 6, Plate LXVIII., pass through the centre of the tissue of the trabe- 
culse in which they originate, through the meshes of the outer wall near the centre of the 
stars, and spread in all directions in the mud. As each spicule is invested by a sheath of 
very soft sarcode, the whole of the mud to a distance round the sponge is glairy, as if it 
were mixed with white of egg. Accompanying the long simple fibres there are usually in 
each fasciculus from three to eight or ten of the hamate spicules (Plate LXVIII. fig. 5) ; 
the bihamate form (figs. 5 & 5 a) is much the most common. Some of these spicules 
end close to the outer network of the sponge (Plate LXX. fig. 1, a), while others accom- 
pany the bundle of fibres to a distance of several millimeters beyond it. The delicately 
spined spicule (Plate LXVIII. fig. 4) also occurs in these fasciculi, but comparatively 
rarely. 
The middle zone of the sponge is nearly free from projecting spicules, and the stars 
are there very regular and distinct ; but on the upper third, towards the edge of the 
osculum, the tissue becomes altogether closer and more confused, the meshes of the sili- 
ceous network become smaller, and the sarcode fills them up as a finely perforated 
membrane. Here, again, bundles of fibres pass outward from the trabeculae of the 
sponge ; but these are shorter and more rigid than the fibres of the “ beard,” and 
project outwards and upwards at a definite angle, forming a kind of coronet round the 
top of the sponge. Each fasciculus consists of a few (ten or twelve) larger and smaller 
smooth simple spicules, with one or two bihamate spicules, a number of the spined 
spicules (Plate LXVIII. fig. 4), and usually a bunch of the silky fibres (Plate LXVIII. 
fig. 7). These fasciculi, which are about 30 mm long on the shoulder of the sponge, 
gradually shorten towards the osculum, till immediately round its lip there is a close 
continuous vertical fringe, chiefly of smooth spicules about 10 mm high. 
