OF THE SPONGIAD.E. 169 



the inner surface of the crustular clermis. Fio;. 354, 

 Plate XXVIII. 



The organization of this external crust is exceedingly 

 interesting. The outer surface is composed of a uniform 

 thin pellucid dermal membrane, perforated with innumer- 

 able minute pores, variable in their diameter, and ap- 

 parently possessing the power of opening or closing at the 

 will of the animal. Immediately beneath the dermal mem- 

 brane there is a stratum of sarcode of variable thickness in 

 different species, and this stratum is permeated by numer- 

 ous short canals, connecting the external pores with the 

 intermarginal 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, fre- 

 quently assuming, by the contraction of the dermal mem- 

 brane, more or less of a stellated appearance. The proxi- 

 mal extremities of these organs is at the inner surface of 

 the stratum of ovaria, and the distal extremities at the outer 

 surface 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. 



The expando-ternate spicula, which are situated at the 

 distal extremities of the radial fasciculi of the skeleton, 

 diverge slightly from each other from their basal extremi- 

 ties, so that their triradiate heads, when firmly cemented to 

 the inner surface of the ovarian stratum, form a strong and 

 regular siliceous network, the points of the radii of each 

 being cemented by keratode to those of its next neighbour ; 

 and within the area of each of these meshes of the network 

 there is the proximal end of an intermarginal cavity, the 

 diaphragm of which frequently occupies the greater portion 

 of the area, having a much greater diameter than that of 

 the proximal orifice of the cavity, so that when fully opened 

 its orifice is quite equal to that of the intermarginal cavity. 

 The ovaries vary considerably in size in different species. 

 In the adult and prolific condition they have the form of a 



