106 THE SPONGES. 



The entire dermal membrane of the pore surface (Fig. 4, Plate 15) is 

 riddled with the fairly evenly distributed pores, which have a diameter 

 of about 75 /x. But the pores immediately over the ends of the larger 

 canals form areas conspicuous to the eye, and thus, when the whole sponge 

 is examined from the surface, the pores appear to have a localized distri- 

 bution in small rounded or irregularly shaped areas. Some of the main 



afferent canals are formed by the confluence of small, elongated, tubular 

 subdermal spaces. 



On the oscular surface (Fig. 3, Plate 15; Fig. 3, Plate 16), the 

 efferent canals are closed in by oscular membranes, which are commonly 

 perforated by groups of small oscula up to about 10 in number. The 

 groups are of irregular shape, and vaguely outlined, although conspicuous 

 to the eye. With a lens it may be seen that many of them are branched, 

 the arrano-ement indicatino; that the efferent canals themselves sometimes 

 extend out beneath the surface in tangentially spreading branches. In the 

 case of other canals the oscular membranes roofing them in are per- 

 forated by single similar apertures or by small groups of two and three. 

 The individual oscula, whether arranged singly or in groups, have a 

 diameter of 70 to 200 /x. 



The oscular membranes are very thin, and in the immediate neighbor- 

 hood of the apertures contain only spirasters and sparsely scattered 

 microxeas. Between the apertures of a group the microxeas are more 

 abundant, and occasionally a megasclere extends into or through the 



membrane of an oscular area. Between the separate oscula or groups 



of oscula which are visible to the eye, the dermal membrane, Fig. 3, 

 Plate 15, is studded with thin rounded areas 50-75 fx in diameter, 

 representing the roofs of small spaces everywhere present beneath the 

 membrane. These thin areas contain spirasters and sparse microxeas, and 

 are frequently perforated. Thus the plan of the canal apertures on the 

 oscular face is much like that on the pore face. 



The choanosome, which looks rather dense to the eye, is permeated by 

 abundant small canals, which reduce it to thin trabeculae. Collenchyma 

 is scanty. The ectosome is thin and feebly marked ; differentiated from 

 the choanosome only by its pores and skeleton, and by the absence of 

 flagellated chambers. 



The choanosome contains an abundance of deeply staining cells similar 

 to those described by Sollas (1888, p. 81) for Poccillastra sckulzci The cell 



