AND PHYSIOLOGY OF THE SPONGIAD^. 
1111 
The free condition of the fasciculi of the skeleton connects this genus in some degree 
with the Halichondroid genera of sponges, but there are none of the species of those 
genera in w^hich the fasciculi of the skeleton are separate from each other. The nearest 
allied genus in that direction appears to be Hijmeniacidon. Plate LXXIV. fig. 3, and 
Part II. Plate XXXI. fig. 8. 
Suborder II. Spiculo-membranous skeletons. Composed of interstitial membranes 
having the skeleton- spicula irregularly dispersed on their surfaces. 
The prominent character of this Order is, that the spicula of the sponges composing 
it do not assume either the radiate, fasciculate, or reticulate structural arrangement, 
the distribution of the spicula on the interstitial membranes being without any approxi- 
mation to order. 
Hymexiacidojt, Bowerbank. 
Skeleton without fibre ; spicula without order, imbedded in irregularly disposed mem- 
branous structure. 
In Hymeniacidon the spicula are subordinate to the membranous structure, they 
follow its course and are imbedded without order on its surface. The contrary is the 
case in Halichondria. The network of spicula in that genus, although irregular, is 
decidedly the predominant structure, and the membranous tissues are secondary to it 
and exist only as interstitial organs. The larger and stouter of the spicula in Hymenia- 
cidon, although dispersed amid the slender ones, may be considered as the representatives 
of the skeleton-spicula, while the slender ones are truly those of the membranes. 
In some species the interstitial tissues are constructed diffusely, as in II. caruncula, 
while in other species, as in IL suhereum [Halichondna subcrea, Johnston) and a few other 
closely allied species, they are more than usually compact, so that in the dried state the 
texture of these sponges is very like that of fine hard cork. From this peculiarity of 
their appearance in the dried condition, and the exceeding compactness of their structure, 
I was formerly inclined to believe them to be generically different from the great mass 
of the species of Ilymeniacidon.^ and I accordingly inserted them in the list of British 
sponges, published in the Report of the Dredging Committee in the Reports of the 
British Association for 1 860, under the titles of Ilalina suherea, IL ficus., &c. ; but a closer 
examination of their internal structure has convinced me that their only real difference 
from the other species of Ilynieniacidon is in their greater compactness of skeleton- 
structure, and I have accordingly removed those species to the genus Hymeniacidon. 
In the greater number of the species of this genus the tension spicula are of the 
same form as those of the skeleton, and are only to be distinguished from them by their 
greater degree of tenuity ; but in a few of the known species they are different both in 
size and form. 
The mode of propagation in all the species in which I have found the reproductive 
organs appears to be by internal gemmulation. In II. carnosum and several other species 
MDCCCLXII. 7 M 
