PORIFERA. II. 207 



most frequently closely grooved, and the grooves are separated by curling or meandering walls. Some- 

 times the walls run chiefly in the longitudinal direction of the sponge, and the surface may get an 

 appearance, as if longitudinal fibres were running below it. Sometimes the pore surface is fairly even, 

 the walls not being raised; this, I suppose, is owing to the degree of contraction of the sponge. The 

 surface is otherwise finely shaggy from projecting spicules. The dermal membrane is a rather thin 

 film, supported by dermal spicules. Oscula and pores: As mentioned above, one surface is the oscular 

 surface, the other the pore surface. On the oscular side rather close-set, circular openings of various 

 sizes are seen. When the dermal membrane is removed, oscular canals are seen, of an average width 

 of 3 mm ; this size, therefore, is the largest one attained by the oscula, but they are seen in all degrees 

 of closing. When they are closed or almost closed, the closing membrane is generally somewhat sunk 

 into the canal, so that a more or less deep, about circular groove is seen. From the oscular apertures 

 canals pass into the sponge, partly tolerably horizontally through the leaf, partly also running in other 

 directions. In the upper part of the sponge they may often be seen rather distinctly to run downward 

 and inward in the leaf. On the pore side the pores are found in groups over the subdermal cavities 

 or the openings of the incurreut canals; they were measured from quite small ones up to a diameter 



of 0'12 mm . 



The skeleton. The dermal skeleton consists of peuicillate bundles of dermal spicules supporting 

 the dermal membrane. The bundles are partly erect, partly more or less recumbent. On the pore 

 side especially, erect bundles are found in the tissue or the walls between the canals or the subdermal 

 cavities; from the edges of these cavities fibres of dermal spicules pass under the membrane distended 

 over the mouth, and these fibres are then more or less horizontal, but send off spicules into the 

 membrane; by this arrangement the structure arises again here which has been mentioned repeatedly 

 in other species, viz. that the membrane or pore sieve distended over the mouths of the canals, is by 

 the mentioned fibres divided in areas, in which the pores are then lying. On the oscular side the 

 structure is about the same; short fibres of dermal spicules likewise stretch from the edge of the 

 oscular canal, radiating into the membrane and pointing towards the oscular aperture. The main 

 skeleton is a mostly polyspicular network, which is, however, rather irregular. Longitudinal fibres are 

 found, passing up through the leaf and bending towards the surface, but their course is somewhat irre- 

 gular. The spicules placed between them are mostly single. They are placed rather closely and quite 

 irregularly, so that the meshes are irregular, by which means the whole network gets a very irregular 

 appearance. Sometimes the longitudinal fibres may be seen especially distinctly to bend towards the 

 oscular side, which is, perhaps, owing to the fact that they partly follow the course of the canals. 

 Sometimes, on the other hand, the skeleton is far more irregular, so that longitudinal fibres are almost 

 not to be traced. In the nodes of the skeleton a rather copious, but white and clear mass of spongiu 

 is found. 



Spicula: a. Megasclera. 1. The skeletal spicules are styli; they are slightly curved, some- 

 times through their whole length, but most frequently the curve is nearest to the head-end; more 

 rarely the spicules are straight. The point is rather short or middle long, it is sharp and oftenest 

 bounded by straight lines. Their length is 0-57 — 072""", and the thickness is 0014— o - oi8 mm , most 

 frequently the longest ones are not the thickest ones. Developmental forms occurred singly; the finer 



