102 THE SPONGES. 



aperture of varying diameter. The depressions on the oscular face (Figs. 6 

 and 8, Plate 14) are 500-700 /x in diameter, and 1.0 to 1.5 mm. apart. 

 The depressions on the pore face (Fig. 5, Plate 14) are somewhat larger 

 and farther apart than on the oscular face, measuring commonly 700-900 fi 

 in diameter, and lying about 2 to 3 mm. apart, but are not so deep, being 

 in places exceedingly shallow. The membranes lining the depressions on 

 the oscular face are the oscular membranes, the apertures themselves, 

 which are in various stages of contraction, representing the oscula. The 

 membranes lining the depressions on the opposite face are the pore mem- 

 branes, the apertures, which are likewise in various stages of contraction, 

 representing the pores. 



The main afferent and efferent canals are essentially alike. They have 

 a diameter of 500-700 /x, and pass about vertically into the interior from 

 the corresponding surfaces, on which each is covered in by a pore or 

 oscular membrane respectively. Sections vertical to the oscular surface 

 of the sponge, and passing longitudinally through the efferent canals, are 

 shown in Figs. 3 and 7, Plate 14. A section vertical to the pore surface, 

 and passing through an afferent canal, is shown in Fig. 4, Plate 14. A 

 part of the oscular surface, showing three oscular membranes, is repre- 

 sented in Fig. 6, Plate 14, and a part of the pore surface, including a 

 pore area, in Fig. 5, Plate 14. 



The walls of the main afferent and efferent canals are collenchymatous, 

 and exhibit numerous transverse circular ridges. These often project, 

 especially in the neighborhood of the sponge surfaces, a considerable 

 distance into the lumen of the canal, appearing as septa perforated by 

 round apertures. Similar incomplete septa often separate the lateral 

 branches from the main canal. The aperture in the septum may some- 

 times be very small, as shown in the lower half of Fig. 4, Plate 14. In 

 the neighborhood of such small apertures debris consisting largely of 

 shells of Foraminifera is sometimes found collected. It may well be asked 

 how heavy bodies of this kind are moved through sponge canals. The 

 appearance of the internal septa suggests that in them as in the surface 

 membranes the apertures may be closed and opened. Possibly the aper- 

 tures open suddenly, and the contents are passed on from one chamber 

 into another, either as a result of an existing difference of pressure on the 

 two sides of the septum, or as a result brought about by simultaneous 

 contraction of the canal wall. 



