88 DR. J.S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jan. 28, 
whole of its surface; they rarely exceed a line in diameter; the 
smaller ones are frequently simple orifices, the larger ones are slightly 
elevated and marginated. The pores are not visible without the aid 
of considerable microscopical power; with about 100 linear their 
structure exhibits an exceedingly beautiful appearance. They each 
occupy an area formed by the intermingling of the elegant foliations 
of the ternate connecting spicula ; and each little porous area is fur- 
nished with a regular fringe composed of a single series of the small 
dermal tension-spicula, which, projected from its margin inwards, 
meet at about the centre of the space, forming a complete defence 
against the incursions of any minute enemy ; in the dermal mem- 
brane around, the minute tension-spicula are closely and irregularly 
matted together (Plate IV. fig. 5). : 
When we view a section of the sponge made at right angles to its 
surface, the structural peculiarities of the expansile dermal system 
of this tribe of sponges are very beautifully displayed. The outer 
surface is densely covered with the terminations of the ternate spi- 
cula of that organ, and again with the dermal membrane and its 
closely matted tension-spicula. Immediately beneath we see the 
pendent shafts of the ternate spicula, more or less clothed with minute 
acerate spicula, and with the proximal terminations of the shafts 
cemented by keratode to projecting portions of the fibre of the rigid 
skeleton, the surface of which is covered by a stratum of membra- 
nous structure, abundantly furnished with minute acerate spicula ; 
the space between this surface-membrane of the rigid skeleton and 
the under surface of the expansile dermal system forms a large ca- 
vernous or crypt-like cavity supported by innumerable pillars at 
about equal distances from each other. 
The arrangement of the fibres of the rigid skeleton have all the 
complete irregularity of a Dactylocalgz, and there is not the 
slightest approach in any part to the confluent radial structure of an 
Iphiteon. ‘There are a few comparatively large acerate spicula dis- 
persed amid the reticulations of the rigid skeleton ; they are about 
four or five times the length of the dermal ones, and they are not 
frequently to be seen iz situ. The connecting spicula are exceedingly 
beautiful objects. They are very variable in size and structure ; and 
no two of them are alike in the mode of the foliations of their ternate 
radii, which are evidently modified to meet the necessities of the in- 
termingling of their terminations, so as to secure a strong and elastic 
covering to the interstitial cavity beneath, and at the same time to 
produce abundant spaces for the porous areas of the dermis of the 
inhalant system. The structural aspect beneath the exhalant sur- 
face is very different from that of the inhalant one: here we find, as 
might be expected, large cavernous spaces for the reception of the 
effete streams from the rigid skeleton beneath, and, instead of the 
regular crypt-like form with its numerous minute pillars, we have 
elongated extensive spaces, the sides of which are, to a great extent, 
composed of irregularly disposed large acerate spicula imbedded in 
membranous structure; the shafts of the connecting spicula above 
are some of them connected with the parietes of the cavernous spaces, 
