332 DR. J.S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [ May 13, 
its delicate structure, and the total absence of dermal spicula, it is 
evident that it has formed no part of the expansile dermal system, 
and that it is truly the enveloping membrane of the rigid skeleton 
of the sponge thickly covered by sarcode. 
Whether these minute molecules are the basal vesicies of the 
ciliary system is a question of considerable interest, to be hereafter 
determined by naturalists who have the opportunity of examining 
these interesting sponges fresh from their native element. Their 
situation and general character are very similar to the homologous 
organs in Gruntia compressa, and their position in I. Ingalli is just 
that in which we should expect to find the ciliary system. We 
cannot hope to find any cilia remaining under such circumstances ; 
those of Grantia compressa and other nearly allied species are rarely 
visible, except during the life of the animal and while in a state of 
activity. 
In a small piece of the membranous structure of this sponge 
which I received from my friend Mr. Ingall, in March 1860, the 
appearances presented are widely different from those of the mem- 
brane I have described above. The colour and the sarcode are very 
similar; but there is a total absence of the minute spherical bodies. 
The field of view presents a very confused appearance. Numerous 
long, slender, and flexible attenuato-acerate spicula are confusedly 
matted together, and amongst them there are a considerable number 
of large rectangulated hexradiate spicula with radii of equal length, 
gradually attenuated from their proximal to their distal terminations ; 
and amidst this complicated mass there are innumerable spinulo- 
quadrifurcate hexradiate stellate retentive spicula. 
From what part of the sponge these portions of its structure have 
been derived it is difficult to conjecture ; but it is evident that there 
are other forms of spicula than those we have observed im siéu that 
belong to it, and that, although the spinulo-quadrifurcate retentive 
spicula are rather abundant in the interstices of the rigid skeleton, 
there are other parts of the sponge in which they are crowded te 
such a degree as to be innumerable. 
The general appearance of the spinulo-quadrifurcate hexradiate 
spicula is very like that represented by fig. 2, P]. XXIII.; and I have 
chosen a mutilated specimen which has only three of its primary rays 
remaining as best calculated to display its quadrifurcate structure. 
The configuration of the rigid skeleton is decidedly that of an Iphiteon 
of a somewhat delicate structure. The skeleton-fibres near the 
surface are rather strongly tuberculated; but those of the interior 
are very much less so, and in some parts they are almost smooth. 
The mouths of the incurrent canals on the surface of the rigid skeleton 
are numerous and frequently closely adjoining each other, the sepa- 
ration often not exceeding half of their own diameter. The auxi- 
liary fibres at some distance within these canals are frequently 
abundant and much complicated in structure. They are stout, very 
rugged, with irregularly disposed tubercles, and their free termina- 
tions are spinulate. The rectangulated hexradiate interstitial spi- 
cula in situ are few in number, and their radii are gradually 
