72 DR. J.S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jan. 28, 
of which I have been treating at right angles to their surfaces and 
then mount them in Canada balsam without previously immersing 
them in water, we frequently find portions of their surfaces in which 
the expansile dermal membrane has dried without having come into 
close contact with the rigid skeleton beneath it, and we see the shafts 
of the connecting spicula pendent from the inner surface of the der- 
mal membrane and freely suspended in the intervening space; and 
under these circumstances we also frequently see a secondary thin 
brown dermal membrane closely adhering to the surface of the rigid 
skeleton. Fig. 6, Plate V., represents such a section from Dacty- 
localyx Prattii. 
When the expansile dermal system in Dactylocalyx Prattii has 
been removed, we find the surface of the rigid skeleton closely co- 
vered by this continuous enveloping membrane, which in its present 
condition is closely adherent to the external surface of the rigid ske- 
leton : while this membrane is in its natural state and position, no 
orifices whatever are observable in it; but when it is removed, we 
find immediately beneath it, on the surface of the rigid skeleton, a 
vast number of incurrent orifices of about the average diameter of 
one-third of a line. They are very evenly dispersed at about three 
or four times their own diameter from each other. That the enve- 
loping membrane above them should appear imperforate is perfectly 
natural while the sponge is in a quiescent state; and there is no 
doubt that when requiring nutriment, imbibing-pores would be 
opened above each of the incurrent canals of the skeleton, in the 
same manner as in Geodia and numerous other similarly constructed 
sponges. 
From the lengths of the shafts of the connecting spicula, which 
vary in some species from ;}, to z4, inch, we may estimate tole- 
rably closely the range of the contractile and expansile capabilities 
of the dermal system; and it is exceedingly probable that this space 
contains the aérating organs of the animal, and is truly the homo- 
logue of the large intermarginal cavities that are so numerous in the 
dermal crust of Geodia Barrettii and other closely allied sponges (see 
Phil. Trans. for 1862, pl. 32. fig. 2, a a, p. 788; and ‘ Monograph 
of British Spongiadee,’ vol. i. pl. 28. fig. 354, aa, p. 171). And this 
idea is rendered more probable by the existence of the innumerable 
spherical vesicles on the corresponding membrane of [phiteon Ingalli, 
which have every appearance of being the basal cells bearing the 
vibratory cilia during the life of the animal. 
The most decisive and valuable specific characters are those de- 
rived from the connecting spicula. They vary to a very consider- 
able extent in different species in both size and form ; but whatever 
may be the shape of their apical radii, their mutual connexion is 
always so ordered that not only is there abundant means for their 
combined mass to expand at right angles to the surface of the 
rigid skeleton, but there is always ample room for a great amount of 
expansion and contraction in a lateral direction ; and however com- 
plicated or eccentric may be the radii of their apices when seen 
separately, when za situ they always form a compact reticula- 
