1126 
DE. J. S. BOWEEBANK ON THE ANATOMY 
5. The Skeleton. 
Although the material, mode of structure, and arrangement of the skeleton is more 
especially devoted to the formation of the orders and suborders, it still presents us with 
a sufficient number of minor peculiarities to render it a source of valuable specific cha- 
racters. Thus, as I have already shown, in treating of the relative value of the spicula 
for the distinction of species, the difference in their size affords a good character. The 
closer or more diffuse mode of their arrangement modifies to a great extent the form 
and size of the areas in spiculo-reticulated skeletons, and their habitually greater or 
less number in the thread of the reticulations produces a distinctly different aspect in 
the skeletons of two otherwise closely allied species. The presence or absence of defen- 
sive spicula, the mode of armature, and the forms of the defensive and other auxiliary 
spicula also afford a very extensive and valuable series of specific characters. In the 
kerato- and siliceo-fibrous sponges there are peculiarities of a similar description, such 
as the presence of a reticulo-fibrous sheath, as represented in figs. 9 & 10, Plate XXVIII. 
Part II., or the possession of spines or tubercles of various forms, as represented in the 
same Plate, figs. 7 & 8, or of extraordinary modifications for prehension, as in the 
cidarate siliceo-fibrous skeleton, represented also in the same Plate, fig. 12. These and 
other similar structural peculiarities afford a series of characters which are usually of a 
permanent and very striking description. 
6. The Interstitial Membranes. 
The peculiarities of the interstitial membranes consist principally in the shape and 
proportions of their tension spicula, or of the forms and varieties of structure, and mode 
of disposition, of the retentive spicula. The latter class of organs especially present a 
very extensive series of striking characters that are essentially specific. In the genera 
Halichondria, Isodictya, Hymeniacidon, and others containing numerous species, often 
very closely resembling each other in all the principal structural characters, they fre- 
quently, from the strongly marked peculiarities in their form and proportions, present 
most valuable and decisive specific characters. Plate XXVII. figs. 1, 2, 3 & 5. 
In Alcyoncellum and other genera the interstitial membranes are strengthened and 
supported by layers of primitive fibrous tissue, arranged in parallel lines ; and in Sterna- 
tumenia the same fibres abound, but they are not disposed in the same symmetrical 
manner, and in some sponges cellular structures are present in considerable quantities. 
These tissues are all more or less valuable as aids in specific distinction. 
7. The Intermarginal Cavities. 
The intermarginal cavities in the greater portion of the Spongiadse are so indefinite in 
their form as to render but little service in the distinction of species ; but in Geodia, 
Pachymatisma, and a few other genera their structure is very much more regular, and 
their form, proportions, and mode of disposition afford good characters. But although 
of no extensive essential value themselves, their subsidiary ternate spicula present a 
