706 
DR. WYYILLE THOMSON ON HOLTENIA. 
3. The Spicules of the Sarcode .— The different forms of spicule which have been hitherto 
described are stationary, woven into certain definite positions in the texture of the 
sponge. Besides these, three distinct types of extremely minute spicules are imbedded 
in the sarcode of the sponge and evidently follow its movements, thus occupying varying 
positions with reference to the permanent skeleton. The most abundant of these are 
very minute spicules formed on the hexradiate type, but having, as in the case of the 
large spicules of the skeleton, only five rays developed. The four secondary branches 
radiate in the form of a transverse cross, and the fifth projects at right angles, and is 
feathered with diverging spines curved toward the distal extremity of the ray. These 
spicules differ in form and size in different parts of the sponge. Those of the outer 
network (Plate LXVIII. figs. 9 & 9 a) are about 0 mm T5 from point to point of opposite 
rays of the cross. The rays of the cross are stout and rigid, and are frequently slightly 
roughened with small tubercles. The feathered ray averages about T7 in length; it 
is robust, and the spines are numerous and close. 
Spicules of this type are very numerous in the sarcode of the oscular network, but 
they have a somewhat different character. The rays of the cross are slightly longer than 
in the spicules of the outer network, and are usually smoother and somewhat more deli- 
cate. The feathered ray is much longer, averaging about -22 to 0 mm, 35 in length ; it is 
much more delicate, and sometimes slightly curved, and the spines on its surface are 
smaller, less numerous, and more distant (Plate LXVIII. figs. 10 & 10 a). 
In the substance of the sponge, these spicules present many varieties in form. Some- 
times they are very short and thick, with the barbs of the feathered style numerous and 
long, and the rays of the cross strongly tubercled (Plate LXVIII. fig. 11) ; more frequently 
they are altogether very delicate in their proportions, so much so that the rays of the 
cross curve irregularly, and the feathered style with its spines entirely lose their rigid 
and uniform outline (Plate LXVIII. figs. 12, 13, & 14). 
The spicules of the sarcode of the second type are not nearly so abundant. They are 
simple and style-like, slightly fusiform, and come to a fine point at each end ; they are 
about 0 mm, 2 in length (Plate LXVIII. figs. 8 & 8 a). The shaft is covered with extremely 
delicate adpressed spines. These spicules somewhat resemble a small portion of one of 
the long spicules (Plate LXVIII. fig. 4) on a very reduced scale. The last spicule of the 
sarcode is a small “ amphidiscus,” a cylindrical spicule 0 mm, 07 in length, with, at either 
end, a crown of six or eight long recurved hooks. These spicules are very numerous, 
especially so, ranged along the branches of the large spicules of the inner network. They 
vary slightly in form (Plate LXVIII. figs. 15, 16); but the differences do not seem to 
be connected with their distribution in the sponge. 
The general Arrangement of the parts of the Sponge. 
1. The outer wall . — The external surface of the sponge is formed of an open network, 
which presents a very beautiful though somewhat irregular stellate appearance. The 
centres of the stars, in a band round the middle of the sponge, where the star-like 
