AND PHYSIOLOGY OP THE SPONGIADiE. 
1099 
terminating in the distal extremity of the intermarginal cavities. Intermarginal 
cavities separate, symmetrical, subcylindrical ; each furnished with a membranous 
valve at its proximal extremity. 
The genus, as described by Lamaeck*, is so loosely characterized that I have thought 
it better to reconstruct it entirely than to endeavour to amend it. I have therefore 
given a new series of characters, founded solely on its structural and organic pecu- 
liarities. I am acquainted with seven species, all of which perfectly agree in the 
essential generic characters as thus constructed. 
The type specimen of Lamaeck’s Geodia gihherosa in the Museum of the Jardin des 
Plantes of Paris, the organization of which, through the kindness of Professors Milne- 
Edwaeds and Valenciennes, I have had an opportunity of thoroughly examining, is 
unfortunately in so deteriorated a condition in many respects, and especially in regard 
to the dermal membrane and pores, that I have been induced to select G. Barretti from 
which, to a great extent, to describe the interesting and highly organized structures of 
this genus ; and I have the advantage also in this species of having a portion of a speci- 
men which has never been deteriorated by drying, having been pickled in strong salt 
and water immediately on being taken from the sea, by my friend Mr. M'^Andeew, and 
in this state it closely resembles a mass of somewhat indurated animal liver. 
The skeleton is composed of continuous fasciculi of stout long spicula, which in 
massive specimens radiate from the base to the outer surface of the sponge, or, if the 
species be of an elongated form, from the central axis to the circumference, where in 
either case they terminate at the inner surface of the crustular dermis, intermixing with, 
and being firmly cemented to, the shafts of the expando-ternate connecting spicula, which 
are attached to and firmly support the inner surface of the crustular dermis. 
The organization of this external crust is exceedingly interesting. The outer surface 
is composed of a uniform thin pellucid dermal membrane, perforated with innumerable 
minute pores, variable in their diameter, and apparently possessing the power of open- 
ing or closing at the will of the animal. Immediately beneath the dermal membrane 
there is a stratum of sarcode of variable thickness in different species ; and this stratum 
is permeated by numerous short canals, connecting the external pores with the inter- 
marginal cavities which occupy, at nearly equidistant points, the thick stratum of ovaria 
forming the inner layer of the crustular dermis. In dried specimens, the positions of 
the intermarginal cavities are usually indicated on the surface of the sponge by a series 
of dimples or pits, frequently assuming, by the contraction of the dermal membrane, 
more or less of a stellated appearance. The proximal extremities of these organs is at 
the inner surface of the stratum of ovaria, and the distal extremities at the outer sur- 
face of the same stratum ; and this termination has usually a greater diameter than the 
proximal end, which is furnished with a stout contractile diaphragm or pyloric valve. 
* Polyparimn Uberum, camosum, tuberiforme, intus cavum et vacuum, in sicco durum ; externa superbeie 
undique porosa. Poramina poris majora in area unica orbiculari et laterali observata. (Lamarck, Anim. s. Vert, 
2de edit. ii. p. 593.) 
