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



lattice-work, witli meshes which do not attain a width of a ^ mm., and are separated by 

 yet thinner strands, the skin on the rib-like projecting truncate edges exhibits a much 

 more irregular network with polygonal or roundish meshes, 1 to 2 mm. in diameter, and 

 with firm whitish strands passing laterally into a compact marginal fringe about 1'5 to 

 2 mm. in breadth, which forms the boundary between the narrow-meshed quadratic net- 

 work of the sides, and the wide-meshed firmer sieve-network of the rib-like edges (PI. LI. 

 figs. 1, 16). Through both these networks, which difi"er so much from one another, the 

 subjacent cavities are seen, shining through as a labyrinth of united passages. On the 

 superior, somewhat obliquely pyramidal or flatly conical, truncate extremity of the sponge, 

 the -ttdder network, strengthened by firm solid junction-plates, is terminally expanded and 

 united, while the fine quadratic dermal lattice-work is confined to the sides, with the 

 exception of several angular or rounded terminal prolongations, which extend beyond 

 the lateral terminal margin on to the summit. Where some of this lateral dermal 

 sheath has been rubbed off by accident or design, the labyrinthine passages and spaces 

 are directly exposed, and it can be seen how the canals and cavities beneath the narrow 

 C[uadratic dermal network form a connected, continuously distinct anastomosing system 

 of canals, a little finger's breadth thick, which are separated by a wall of 1 mm. thick- 

 ness from another adjacent canal system, which likewise forms a connected network of 

 anastomosing spaces and passages. This second system of passages extends directly under 

 the wide-meshed, more irregular lattice-work, which we have noted on the rounded off 

 lateral edges. The two canal-systems are everywhere separated only by a thin partition, 

 and their canals extend side by side, not only under the external skin, but penetrating 

 inwards, traverse the whole internal body, so that the broad, irregular, longitudinal canal 

 in the axial region is an integral part of that system of passages, which extends below 

 the wider iiTegular network of rounded side margins, and which opens at the superior 

 terminal plate. • This remarkable presence of two completely separated canal systems 

 is represented in PI. LI. fig. 16, in the lateral portion of the sponge from which the 

 external wall has been partly removed, and also in the diagrammatic cross section of 

 the whole sponge in PL LII. fig. 1. 



Through the fine quadratic dermal network which covers the slightly convex lateral 

 surfaces of the body, the water passes first into the subjacent canal system, by which it 

 is carried to all parts of the body, penetrating at length through the partition wall (which, 

 though only about 1 mm. thick, contains the memhrana reticularis and forms the proper 

 parenchyma) into the second system of canals which is in connection with the central 

 longitudinal canal or gastral cavity. Thence the water reaches the exterior by the 

 efferent ducts, namely, either by the wide-meshed oscular sieve-network of the longitudinal 

 side edges, or by the superior terminal region. 



That the relation of these two adjacent, but perfectly distinct, labyrinthine canal 

 systems to the flow of water is as above described, is demonstrable not only from their 



