THE SPONGES. 147 



determined by the character of the main efferent canals, which conditions 

 the arrangement of the oscula and superficial collenchyma. The main 

 efferent canals are numerous, cylindrical, and pass radially into the body 

 from both surfaces. The diameter does not exceed 2 mm., and commonly 

 is 1 to 2 mm. The canals penetrate deeply into the body, and in many 

 cases pass completely through the hody from one surface to the oilier. Tlie 

 oscula are sometimes single apertures, but often the end of the canal is 

 covered in by a fenestrated membrane, including a few, 3 or 4, apertures. 

 When the canal passes completely through the body, at least one end 

 seems always to be covered in with a fenestrated membrane. 



Near the lower surface of the sponge several canals, which open inde- 

 pendently on the upper surface, may unite and thus produce a vestibular 

 space which is separated from the exterior only by the dermal membrane 

 of the lower surface. Such vestibular spaces are abundant in some speci- 

 mens on those parts of the lower surface which seem to have been moulded 

 over an underlying object (Fig. 13, Plate 20), and here appear as de- 

 pressed membranous areas, which are usually elongated, often somewhat 

 meandering. (In the figure they appear darker, the more solid sponge 

 tissue between them reflecting the light better — the sponge being im- 

 mersed.) They open by oscula, in the case of the larger spaces by several, 

 which range from a small size to a diameter of 2.5 mm. In these specimens 

 the vestibular spaces are found only on the lower surfixce. 



The common type of osculum, represented by the single apertures or 

 fenestrated membranes of the canals which open independently on the 

 surface, is in some regions not surrounded by collenchyma (Fig. 12, 

 Plate 20), in other regions is so surrounded (Fig. 3, Plate 20). The 

 oscula again may not be depressed (middle part of Fig. 12, Plate 20), 

 or may be markedly depressed (Fig. 3, Plate 20). The oscula, and 

 associated canals, may in one portion of a specimen be so numerous as 

 to honeycomb the sponge (left of Fig. 12, Plate 20), and in another 

 region (middle of same figure) be comparatively far apart. The oscula 

 are in general more abundant on the upper surface. 



In two of the specimens large parts of both surfaces present a striking 

 modification, which may be referred to as the reticulate modification. In 

 these regions the surface is comparatively smooth and exhibits collen- 

 chymatous areas of a rounded, polygonal shape and 1 to 2 mm. in diameter, 

 separated by narrower tracts of the more solid sponge tissue (Fig. 2, 



