68 THE VOYAGE OF H.M.S. CHALLENGER. 



three faces of the pyramid there is a broad groove ; hence the sponge might also be 

 described as composed of three broad, ascending, divergent lobes, arising from a common 

 base, and united together almost up to their apices. Colour in spirit pale yellow. 

 The texture of the sponge is very firm and dense, but internally it is traversed by 

 wide canals. The dermal membrane is thin and transparent, loaded with mega- and 

 microsclera. The surface is smooth and even, except for numerous irregular cracks which 

 traverse it in every direction. These cracks (PI. XIV. fig. 1, p.a.) form a reticulation 

 all over the surface, except on the summits of the lobes, where they are absent. The 

 efiect thus produced closely resembles that of sun-cracks upon a cake of mud. Some of 

 the cracks are quite closed, others are gaping, and in the latter condition they are seen 

 to be crossed, at a little distance below the general surface of the sponge, by a delicate 

 membrane, while in this membrane, from wall to wall of the crack, run very numerous 

 transverse bands of fibres, distinctly visible to the naked eye. 



Upon examining prepared sections with the microscope the real meaning of these 

 cracks is at once seen. They are pore-areas (PI. XIII. fig. 16). The delicate membrane 

 forming the floor of each is pierced by numerous small holes, the pores, which reduce 

 it to a mere sieve. These pores are about 0"06 mm. in diameter, and lead into large 

 subdermal cavities, immediately underlying the cracks. The terminal branches of the 

 incurrent canal system open out of these subdermal cavities by round mouths about 

 1 mm. in diameter. The bands of fibres above mentioned, which run across in the 

 pore-bearing membrane from side to side of the cracks, stain deeply in borax carmine, 

 and there can be little doubt that they are contractile bands, or, in other words, muscles, 

 whose function is to open and close the cracks and thus to regulate the supply of water. 

 A few pores also occur scattered in the dermal membrane, over the general surface of 

 the sponge. 



Although more properly coming under the head " Skeleton," we cannot here 

 pass over the way in which the edges of the pore-bearing cracks are formed. 

 Each edge is guarded by a bristling row of projecting spicule-points. The spicules to 

 which these points belong are arranged in tufts ; each tuft radiates from a point below 

 the dermal membrane, a little to one side of the crack, and the spicules project obliquely 

 upwards and terminate in the fringe along the edge of the crack {vide PI. XIII. fig. 16). 



The oscida are grouped on the summits of the lobes (PI. XIV. fig. 1, o, and fig. la), 

 and with this fact must be connected the absence of pore-bearing cracks in these regions. 

 There are in all about twenty oscula, averaging in diameter about 4 mm. 



Skeleton. — (a) Dermal; a very dense, felted layer of stylote sjiicules, laid hori- 

 zontally and not arranged in fibres. (J>) Main; a rather irregular but compact 

 reticulation of dense spiculo-fibre, in which one may distinguish main fibres running 

 vertically to the surface of the sponge, and secondary fibres crossing these at right 

 angles. As the main fibres approach the surface the spicules composing them spread . 



