156 THE SPONGES. 



of opposite surfaces interdigitating as in PhaJcellia ventilahriim (Ridley and 

 Dendy, 1887, Plate XLIX. Fig. 3). The canals debouching on the inner 

 surface, efferent canals, open for the most part by single oscula nearly as 

 wide as the canals themselves, and bounded by a narrow rim of oscular 

 membrane. Rarely, instead of a single osculum there is a fenestrated 

 membrane, including two or three apertures. Between the oscula the 

 dermal membrane of this suiface is perforated by scattered apertures of 

 small size, 75-150 ^ in diameter, resembling pores in appearance. Such 

 small apertures are in some places abundant. 



The corresponding canals of the opposite surface, afferent canals, are 

 roofed in by pore-membranes, which in some instances are perforated by 

 from one to a few (3 or 4) pores. But in many cases the membranes are 

 imperforate, the pores doubtless being closed. The open pores have a diam- 

 eter ranging from 75 to 200 /z. The dermal membrane of this surface 

 between the apertures of the large canals is doubtless, in the natural state, 

 perforated by abundant, irregularly scattered, pores. At any rate very 

 numerous short radial canals, 75-150 /a in diameter, abut directly against it. 

 The circular areas of thin membrane covering in such canals are mostly 

 imperforate, but in some cases show an open pore. 



The flagellated chambers are rounded, 32-40 /x in diameter, and their 

 arrangement in the sponge trabeculae indicates them to be eurypylous. 



Spicules. 1. Oxea, 540 /u, x 32 /x, with smaller sizes, Plate 19, Fig. 

 2,a,h,f, g. Spicule may be nearly straight (/), strongly bent (h), slightly 

 bent (^), or evenly and slightly curved {a). It is smooth and tapers from 

 the middle to the moderately sharj) or rounded points. 



2. Small style, 400 /a x 30 /x at the base, Plate 19, Fig. 2, c, h, i. 

 Spicule is smooth and tapers evenly from rounded base to the sharp point. 

 It may be straight {h), slightly bent near the base (z), or sharply bent near 

 the base {c). 



3. Large style, 1275 /ut x 28 /a at the base, Plate 19, Fig. 2, d, e. 

 Spicule is smooth, and tapers evenly from lounded base to the sharp point. 

 It may be nearly straight (e), or conspicuously bent (d). 



Skeletal Arrangement. Wall of sponge is supported by a continuous 

 skeleton, Plate 18, Fig. 10, consisting of a reticulum of spiculo-fibres. 

 which are flattened at right angles to the surface of the sponge, and thus 

 have the character of bands or lamellae, Plate 25, Fig. 1. The meshes 

 are elongated in the direction of radii extending upward from the base 



