244 W. J. SOLLAS OX PHAKETllOSPOXGIA STKAHAXI. 



Oscular Surface. — This (PL XI. fig. 2) is smooth all over and 

 much more even than the other face ; the fibres of its network are 

 broader than those of the interior skeleton ; and their exposed outer 

 sides are flattened so as to lie in the same plane as the general 

 surface of the plate, which thus acquires its noticeable smoothness ; 

 the intermeshes are more or less circular in outline ; and some of 

 them increase at the expense of their neighbours, indicating a ten- 

 dency towards the formation of more specialized oscular openings. 



This surface is always found in the interior of the tubular pro- 

 cesses or closed folds of the sponge, and on that side of the plate 

 which is continuous with the interior of the tubular processes 

 through their proximal openings (fig. 5, e). The other surface oc- 

 cupies just the opposite position, covering the face which extends 

 over the exterior of the tubular processes (fig. 5, a). It is from this 

 relative position of the two surfaces that we are able to determine 

 which is the oscular and which the poriferous one — since in all re- 

 cent tubular sponges the oscules are confined to the inside of the 

 tube, while the outer side is occupied by the pores ; and hence it is 

 clear that the surface we have described above must be the oscular 

 one, while the one to which we now proceed and which covers the 

 exterior of the tubular prolongation, must be the poriferous layer. 



Poriferous Surface. — This (PI. XI. fig. 3) is rough and irregular, 

 owing partly to the fact that the fibres of its network retain the 

 form of those in the interior, and do not undergo any expansion in 

 the plane of the surface, and partly also to the fact that the network 

 is produced irregularly in places above the general surface of the 

 plate, rising and falling in minute ridges and furrows. In ad- 

 dition to the minor inequalities so produced, there are inequalities 

 on a larger scale, such as the long and rounded ridges and fur- 

 rows, the pits and dimples which mark the surface of the speci- 

 men represented in PI. XI. fig. 1 atr andp. The poriferous surface 

 is continued over the edges of the plate, and sometimes intrudes to 

 a slight extent on the oscular face, as is well seen at the mouths 

 of the tubular prolongations, where, the poriferous surface covers not 

 only the lips but dips for a short distance into the interior of the 

 tube, as is exhibited in the following section (Fig. 5). 



Fig. 5. — Diagram section of a tubular prolongation. 



J» u. Poriferous surface. c. Tubular process. e. Oscular surface. 



b. Distal opening or mouth. d. Proximal end. 



