DE. WYVILLE THOMSON ON HOLTENIA. 
707 
arrangement is most conspicuous, are about 10 mm asunder. In the upper portion of the 
sponge, towards the lip of the osculum, the stars gradually diminish in size and become 
less obvious, while close to the lip the stellate arrangement disappears, the meshes be- 
come close and irregular, and the tissue looks like fine grey felt. Towards the base of 
the sponge the network also loses its regularity, but it becomes more coarse and open. 
The stellate effect of the outer wall is due to a multitude of more or less strongly marked 
centres, from which an irregular number of rays diverge. The most strongly marked of 
these centres of radiation form the centres of the prominent stars, but the spaces be- 
tween these, and among their rays, are full of smaller radiating points from 1 to 2 mm 
distant from one another, and having in every way the same character as those which 
are more obvious. 
The centre of each of the large stars coincides with the point of divergence of the rays of 
one or two of the large five-rayed spicules (Plate LXVIII. figs. 1, 2, Plate LXIX. figs. 2, 3), 
and very usually, besides the very large spicules, one or more of the smaller spicules of 
the same class radiate at the same point. The smaller stars indicate the point of radiation 
of these smaller spicules alone. The spicules are placed thus : four rays of each spicule 
spread nearly on the same plane, forming the rays of the star and the supporting frame- 
work of the outer wall of the sponge, while the fifth ray plunges down into the sub- 
stance of the sponge, and forms one of the elements of the remarkable spongy trabe- 
culae which support the outer casing. 
The siliceous rays of one star curve towards and meet the rays of the neighbouring 
stars, and run along parallel with them (Plate LXIX. fig. 2). All the rays of all the 
spicules are thickly invested with consistent semitransparent sarcode, which binds these 
concurrent branches together by an elastic union, and fills up all the angles of the 
meshes with softly curved viscous masses. This arrangement of the spicules, free, and 
yet adhering together by long elastic connexions, produces a strong, flexible, and very 
extensible tissue. 
In the more open parts of the outer wall, the meshes between the spicules are from 
•o to 0 mm, 8 in diameter, and even with a hand-lens these spaces may be seen to be covered 
by an extremely delicate transparent finer netting. This ultimate network, however, 
has no siliceous frame, but consists simply of irregularly inosculating threads of sarcode, 
which rise from the sarcode-sheaths of the branches of the spicules, and stretch, anasto- 
mosing irregularly, across the spaces ; bounding and defining the inhalent pores of the 
sponge (Plate LXIX. figs. 2, 3). There can be no doubt from its appearance that the 
sarcode of this network presents the movement which we find so characteristic of this 
histological element throughout the group. It is soft, very transparent, and of a pale 
horn-colour. It contains scattered through it, separate or in groups, minute brownish 
granules apparently of some form of horny matter, with here and there bodies of more 
definite outline resembling endoplasts, and two forms of spicules. Where the sarcode 
covers the branches of the five-rayed spicules and forms the border of the meshes of the 
larger network, the feathered spicules (Plate LXVIII. fig. 9) are very abundant. The 
transverse crosses are imbedded in the sarcode and, as it were, stand against the large 
