1875.] SILICEO-FIBROUS SPONGES. 505 
work of the dermis, we at once perceive that the spicula when thus 
united form a strong and elastic rectangulated network. This mode 
of combination of the simple sexradiate spicula is perfectly adapted to 
the power of dilatation and contraction that it appears should neces- 
sarily exist in all siliceous sponges, whatever their form may be, 
which have a rigid skeleton. We find these powers existing in all 
the species of Dactylocalyx, as represented in plates v. & vi., 
P. Z. 8. 1869. But in these cases the expansion of the dermis is 
effected by various forms of ternate spicula, connected by the apices 
of their terminal radii, while their shafts are directed towards the 
body of the sponge, so as to allow, not only of a great amount of 
lateral expansion and contraction of the dermal membrane, but also 
of the separation of the dermis from the body of the sponge beneath 
it to a very considerable extent. In all the species of Geodia and 
Pachymatisma we find the same principle existing under various 
modifications. The expansile powers of the dermal tissues are also 
provided for, in the reticulated structures of {sodictya and Halichon- 
dria, by the conjunction and elastic adhesion of the terminations of 
the spicula forming the dermal rete, whether that organ be mono- 
spiculous, as in many species of Isodictya, or multispiculous, as 
in numerous species of Halichondria and several other genera; and 
where no such structures exist the dermal membrane alone is abun- 
dantly elastic, as exhibited in the protrusion of the large excurrent 
orifices in Spongilla, as figured in plate i. in the “ Report on the 
Vital Powers of the Spongiade,” in the Reports of the British Asso- 
ciation for 1857. All these beautiful appliances appear to be com- 
bined in the structure of the dermis of Alcyoncellum; and in addi- 
tion we have the floricomo-sexradiate forms terminating the distal 
apices of the dermal expansible arrangement of spicula, as defences 
of the external surface of the dermal membrane against the minute 
enemies, while the chevaua-de-frise forms beneath are an ample and 
effective defence against the more powerful depredators. 
The spicula of the expansile dermal tissues vary in structure to 
a considerable extent in the different species of sponges in which 
they occur. In Geodia Dysoni they assume the form of simple 
patento-ternate spicula, their distal terminations being all in the 
same plane, their radii meeting and overlapping each other more or 
less, as represented in figs. 4 & 5, plate iii., P. Z.S. 1873; or they 
occur as bifurcated patento-ternate ones, as represented in figures 
3 & 4, plate ii., P. Z. S. 1873, inthe dermis of Geodia perarmatus. 
In the similar organs of Daetylocalye Pratti their terminal radii 
are flattened and contorted to a considerable extent, as shown by 
figs. 9, 10, 11, plate v., P. Z. S. 1869; and in the same plate the 
radii of these spicula are expanded into beautiful foliations in the 
dermis of Dactylocalyx M‘Andrewii, as represented in figures 2, 3, 
& 4. But however different their forms may be, their office in the 
expansile dermis of each sponge is precisely the same, and their long 
basal shafts are pendent, as represented in the section at right angles 
to the surface of Dactylocalyx Prattii in plate v. fig. 6a, P. Z. S. 
1869. In all these cases the same design, with variations adapted 
