CHAP. 1X. | THE DEEP-SEA FAUNA. 417 
As I have already said, the most remarkable new 
forms are referable to the group which seems to be, 
in a sense special to deep water, the Hexactinellidee. 
I have already (p. 70) briefly described one of the 
most abundant and singular forms belonging to this 
order, Holtenia carpentert; and all the others, 
though running through most remarkable variations 
in form and general appearance, agree with Holtenia 
in essential structure. In the Hexactinellide all the 
spicules, so far as we know, are formed on the hex- 
radiate plan; that is to say, there is a primary axis, 
which may be long or short, and at one point four 
secondary rays cross this central shaft at right angles. 
Very often one-half of the central shaft is absent or 
is represented by a slight rounded boss, and in that 
case we have a spicule with a cross-shaped head, a 
very favourite form in the manufacture, defence, and 
ornament of the surface layer of these sponges ; and 
often the secondary rays are undeveloped: but if 
that be so,—as in the long fibres of the whisp 
of Hyalonema,—in young spicules and in others 
which are slightly abnormal, four little elevations 
near the middle of the spicule, which contain four 
secondary branches of the central canal, maintain the 
permanence of the type. In many of the Hexac- 
tinellidze the spicules are all distinct, and combined, 
as in Holtenia, by a small quantity of nearly trans- 
parent sarcode; but in others, as in ‘ Venus’s flower- 
basket,’ and the nearly equally beautiful genera 
Iphiteon, Aphrocallistes, and Farrea, the spicules 
run together and make a continuous silicious net- 
work. When this is the case the sponge may be 
boiled in nitric acid, and, all the organic matter 
EE 
