126 Transactions of the Society. 



one will have no difficulty in explaining why a dermal layer was 

 found here and nowhere else — not in a single excurrent canal, nor 

 on the sides of the exterior grooves ; although, had it at any time 

 existed in these places, it must almost certainly have left some trace 

 of its existence behind. The truth is, the dermal layer must have 

 been confined to the general surface of the sponge, and covered the 

 walls of our tubule, because these were originally a part of the 

 general surface, and only by accident, as it were, came to assume a 

 tubular form. When the specimen was cleaned the dermal layer 

 would readily be removed from exposed surfaces, but would easily 

 escape destruction in this secluded recess. The absence of a dermal 

 layer from the sides of the grooves on the under surface is most 

 noteworthy, and leads one to infer that the dermal layer on the 

 under surface was continued from ridge to ridge, so as to roof over 

 the intervening gullies without in any case dipping into them. 



The piece of the sponge exhibiting the dermal layer was care- 

 fully cut out and variously mounted for microscopical examination. 



If we commence our observation of a transverse section from its 

 outermost face, we shall see first the distal ends of a number of 

 acerate spicules, which when traced inwards for a distance of about 

 a quarter of an inch, are found to enter, normally to its surface, the 

 skeletal network of the sponge, penetrating through its meshes for 

 about the same distance as they project beyond it. Next we find 

 just outside the skeletal network a dermal layer of sexradiate 

 spicules, each with four long horizontal arms extended in the plane 

 of the dermal layer, and witli the two remaining arms at right 

 angles to it, the distal one short and frequently aborted, the 

 proximal one descending perpendicularly into the meshes of the 

 skeleton like a little rootlet into the soil. The horizontal arms do 

 not appear to be arranged into a regular square-meshed network. 



Beneath the dermal layer we reach the outermost layer of the 

 reticulate skeleton, consisting of framework spicules only just 

 connected together by siliceous cement. The skeletal layer succeed- 

 ing this is still very young, so that its fibres still retain an open 

 lace-like character, not having yet become filled up with the 

 siliceous deposit, which subsequently in the third or fourth layers 

 renders them solid throughout. In the third and fourth layers then 

 the fibres have assumed the form of solid homogeneous threads 

 which only differ from those of the quite adult skeleton by their 

 greater smoothness and less abundant tuberculation. 



Acerate fusiform spicules (Plate VII., Figs. 1,3, and 18). — These 

 are cylindrical in the middle and taper very gradually towards each 

 end, till they terminate in extremities of remarkable tenuity. The 

 longest complete example measured i" in length and 0"0015" in 

 breadth ; but these dimensions may be slightly exceeded in some 

 other cases, though one cannot say so definitely, owing to the fact 



