THE SPOXGES. 103 



The oscular membranes are distinctly outlined. They contain spirasters, 

 and are without or with only a few scattered microxeas, while the dermal 

 membrane in general is densely filled with microxeas. The majority of 

 the oscula are closed or nearly closed, the membranes exhibiting near the 

 centre a rounded patch which appears dark with transmitted light, and 

 which sometimes includes a minute aperture (Fig. 6, Plate 14). In 

 radial sections through closed canals (Fig. 7, Plate 14) it is seen that 

 the oscular membrane is produced inward into a plug-like process which 

 is marked by a dense axial streak of spirasters. The plug of course cor- 

 responds to the dark patch in the surface view of the membrane. The 

 appearances indicate that the oscula are provided with strong sphincters, 

 the contraction of which closes the osculum and produces the plug-like 



projection. The pore membranes and pores (Figs. 4, 5, Plate 14) are 



essentially like the oscular membranes and oscula. 



Collenchyma is abundant throughout the sponge, and contains many 

 granular cells with fine processes. It is, moreover, transversed by abun 

 dant fine fibres, which branch and anastomose (Fig. 1, Plate 14). 



The ectosome is collenchymatous, and distinctly developed, although 

 it varies in thickness. Measured between the vertical canals, it is 350- 

 400 fjL thick at the pore surface, 510 /a thick at the periphery of the sponge 

 800 (J. to 1.0 mm. thick at the oscular surface. 



The flagellated chambers are eurypylous and about 50 fx wide. The 

 chambers (Fig. 1, Plate 14) show the peculiar structure known as 

 SoUas's membrane, described by Sollas (1888, pp. xxxvi-xxxvii) and 

 Dendy (1888, pp. 18-21). The fenestrae in the membrane have for the 

 .most part sharp boundaries. Through some of them the nuclei and pro- 

 toplasmic masses on the boundary membrane of the chamber may be seen. 

 The collar cells are not well preserved, but fine strands may be traced 

 running from the boundary membrane to Sollas's membrane. The distance 

 between the boundary membrane and Sollas's membrane varies consider- 

 ably, and possibly when the distance is great the two membranes are not 

 in their natural positions. 



Meffascle?-es. 



1. Oxea. a. Smooth, slightly curved, or bent spicule, tapering at each 

 end, 2.5 mm. x 85 /x to 5 mm. x 135 /t. Spicule is abundantly scattered 

 through the parenchyma ; also abundant just beneath the dermal mem- 

 brane on both oscular and pore faces, here lying tangentially and to- 



