90 DR.J.S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jan. 28, 
rent to the outer portion of the rigid skeleton. When the back of this 
specimen was presented to the eye, this membrane was seen to be 
abundantly supplied with large, long, flat fasciculi of slender acerate 
tension-spicula. The minute short fusiformi-cylindrical spicula were 
as profusely scattered over the surface of this membrane as on the 
external dermal one. 
The porous system, especially when we view its inner surface, isa 
most beautiful object for the microscope. The interlacing radii of 
the large patento-ternate connecting spicula form a beautiful series 
of round or oval areas, each containing from one to four or five large 
pores, the greater portion of which were open ; and the dermal mem- 
brane on which they exist is beautifully freckled with innumerable 
minute, entirely spined fusiformi-cylindrical spicula, so clesely packed 
together as to completely obscure the surface of the membrane, while 
the acutely conical shafts of the connecting spicula are seen at re- 
gular intervals projected towards the eye. A portion of this beau- 
tiful membrane is represented by fig. 8, Plate V. 
The expansile dermal system is admirably displayed in this sponge 
by a section at right angles to the surface from almost any part of 
it. In some portions of such a section the dermal surface is closely 
pressed on to the surface of the rigid skeleton, while in others it is 
seen more or less separated from it, forming a cavity above it, into 
which the shafts of the connecting spicula are projected towards the 
surface beneath, as represented by fig. 6, Plate V. 
The irregularly furcated patento-ternate connecting spicula are 
singular in their structure, and very characteristic of the species. 
No two of them are precisely alike, either in size or form ; the ec- 
centricity with which the radii are projected from the head of the 
shaft and the exceedingly variable mode of their ramifications are 
not a matter of chance, but they are evidently influenced by the ne- 
cessities of their combinations with each other in forming the dermal 
network and porous areas; for if we view them in situ, we observe 
no points straying from the lines of combination, but the whole of 
their radii are locked together so as to form a compact but expansile 
network for the support of the dermal membrane and the formation 
of the porous areas. 
The interstitial membranes filling the areas of the network of the 
skeleton are very translucent, and would scarcely be visible when 
immersed in Canada balsam, if it were not for the minute, short fusi- 
formi-cylindrical spicula which are dispersed over their surfaces. 
These spicula, though exceedingly minute, afford very decisive spe- 
cific characters. They are dispersed, more or less, over every part 
of the membranous structures, but more especially on the dermal 
membrane and the enveloping membrane of the rigid skeleton, which 
tissues they completely cover. They require a power of from 700 
to 1000 linear to define their structural characters in a satisfactory 
manner. They vary considerably in size; one of the largest that 
I measured was 5); inch in length, and ;;4,, inch in diameter ; 
one of the smallest measured ;2.; inch in length, and z5);5 inch 
in diameter. 
