THE SPONGES. 133 



meshes of the dermal reticuhmi of the upper surface numerous afferent 

 canals the larger of which measure 80-120 jx in diameter, and which in 

 many places are separated only by thin partitions 10—50 /x thick, abut 

 against the dermal membrane. The pores on this surface, which in some 

 places are closed, but in many regions are open, measure 20-40 fjt, in 

 diameter, and lie thickly crowded in the meshes of the dermal reticulum. 

 Those overlying the more conspicuous afferent canals seem with a low 

 magnification to form rounded pore areas, which have about the same 



diameter as the canals. The dermal membrane of the under surface 



is in places riddled with pores, but elsewhere pores appear only here and 

 there, probably owing to closure. 



Spicules. 3Iegascleres. 1. Oxea (Fig. 11, Plate 17), smooth, slightly 

 curved ; points usually sharp, occasionally rounded off, one end rarely 

 strongylate. Spicule abundant. Common size is about 320 fi x 2{j fx, 

 although smaller sizes occur grading down to spicules only 150 /u, x 2-3 /x. 

 The latter are probably young stages in the development of the character- 

 istic oxea. 



Microsderes. 2. Sigmata (Fig. 11, Plate 17), 18 /x long by something 

 less than 2 /a thick, are abundantly scattered through the parenchyma and 

 in the dermal membrane of both surfaces. 



Skeletal Arrangement. In the main skeleton the oxeas are arranged in 

 spiculo-fibres, and are also scattered without order between the fibres. 

 The spiculo-fibres, which are often very loose and best described as tracts, 

 form a confused irregular reticulum, the meshes of which differ greatly 

 in size (Fig. 1, Plate 22, a section vertical to surface). The fibres are 

 of varying thickness, from 200 /x, representing about 10 rows of spicules 

 (as seen in optical section), to 80 /x, or even thinner. They consist of 

 spicules arranged for the most part lengthwise in the fibre, and held 

 together by a very small amount of spongin. In the body of the fibre 

 the spicules are pretty densely packed, becoming loosely arranged at tlie 

 surface. Thus the fibres in general are not sharply separated from the 

 scattered spicules. 



The fibres may be fairly compact, and the spicules which are scattered 

 in the meshes rather few in number. In such spots the reticular nature 

 of the skeleton is obvious. In other spots the fibres are looser and the 

 scattered spicules more abundant, and the reticular nature is obscured. 

 In still other spots the reticular nature is practically lost, there being in 



