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■ 



THE SPONGES. 



157 



' 



un- 



some cases 



toward the free edge of the sponge. Thickness of skeletal lamella (i. e. its 

 narrower cross diameter), about 375 p. Meshes of the reticulum frequently 

 about 1800 /x x 500 /x. The reticulum may be regarded as a system of 

 upwardly extending, branching fibres, the flat surfaces of which are con- 

 nected together by anastomoses. 



The flattened skeletal lamellae vary in character. They may extend 

 nearly through the sponge wall from one surface to the other, or only a 

 part of the way (Plate 25, Fig. 1). The lamellae may be quite 

 branched in the plane in which they are flattened (Plate 25, Fig. 4), or 

 they may be branched in this plane (Plate 25, Fig. 3). In 

 the lamella is so branched as to be vaguely divisible into a main fibre and 

 secondary fibres extending out from it to the surface. The lamellae in the 

 upper part of the sponge are more commonly branched in the plane of flat- 

 tening than is the case in the lower part of the body. 



The skeletal lamellae are composed largely of oxeas (Spicule 1), closely 

 packed and interlaced. From the general surface of the lamella project 

 abundant short styles (Spicule 2), while from the edges' of the lamellae that 

 are adjacent to the surfaces of the sponge, numerous long styles (Spicule 3) 

 project (Plate 25, Figs. 1, 3, 4). It is these latter spicules which pro- 

 trude beyond the surface of the sponge and give to the latter its hispid 



character. The spicules of a lamella are united together by pale 



spongin. The spicules and spongin do not form a continuous solid mass. 

 Nevertheless the spicules are so crowded that only small rounded gaps are 

 left unoccupied by either spicules or spongin. 



The peduncular part of the sponge is occupied by a strong massive 

 skeleton with which the skeletal lamellae are continuous. This peduncular 

 skeleton has the same character as one of the skeletal lamellae. It consists 

 of closely and irregularly strewn spicules, chiefly oxeas, so cemented to- 

 gether by spongin as to produce a nearly continuous mass, which is exca- 

 vated only here and there by areolae. The spongin is more abundant than 

 in the skeletal lamellae. 



Comparative. The arrangement of the skeletal framework in this species 

 is very different, although derivable from that in the well-known P. ventila- 

 brum (Johnston). In the latter only the middle plane of the lamella, which 

 may be 4 mm. thick, is occupied by the reticulum of longitudinal fibres. 

 From these, long loose bundles of spicules pass radially to the opposite surfaces 

 (Bowerbank, 1864, p. 367, Plate XXXIII.). In P. lamelligera a flattened 





