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



by the fact that the perforated meshes alternate (with tolerable regularity) both 

 longitudinally and transversely with those which are closed (PI. IV. fig. 2). 



The number of transverse ridges projecting inwards amounts to sixty or eighty in a 

 full-grown specimen. I have counted thirty longitudinal ridges on the ujjper portion, 

 and about twenty in the lower, which agrees tolerably well with the numbers given by 

 Marshall. The increase in the number of longitudinal ridges in the upper part of the 

 tube is due to the splitting which here and there occurs. 



The watch-glass-like, arched, terminal sieve-plate consists of a lattice-work of laterally 

 compressed ridges of various thickness, which, though exhibiting no very regular 

 arrangement, yet suggest a wheel-like reticular structure. One can distinguish, at least, 

 three or four main beams which are approximately circular and several which extend 

 radially. These form the primary meshes, which are again divided by narrower and less 

 prominent ridges. Here and there a broader plate is formed in the network, as if by the 

 confluence of the stronger beams. 



At the lower end of the body the longitudinal bundles of siliceous fibres gradually 

 emerge on the surface, and breaking up into separate spicules, form the basal tuft. This 

 has a length of from 4 to 8 cm., is tube-like in its upper portion, but towards the lower 

 end becomes brush-like through divergence of the component fibres. This tuft accord- 

 ingly encloses a central inversely conical cavity, into which the extreme lower end of the 

 lattice-like skeleton of the tube-wall extends downwards for a variable distance. In all 

 full-grown specimens I found that this extreme end of the tube was dead, and at a 

 distance of several centimetres from the terminal opening, which is from 1 to 2 cm. 

 broad, the end of the tube was devoid of aU soft tissue, in fact macerated and generally 

 filled with a firm stojiper of mud. The younger the specimen examined, the better was 

 the preservation of the lower end of the tube, and the narrower the terminal opening. I 

 was, however, unable to discover, in any of the specimens at my command, any "pointed 

 terminal cone, formed from the longitudinal and spiral strands of the parietal tissue," 

 such as Marshall has observed in a very young specimen, and has designated the " inferior 

 sieve-plate." 



In uninjured specimens whose soft parts had been well hardened by being preserved 

 in absolute alcohol, no external openings except the parietal gaps could be seen with the 

 naked eye. The sponge was of a pale yellowish-grey colour. The consistence of the 

 soft tissue which covered the skeletal framework in a somewhat thin layer resembled 

 that of hread crumbs, while Wyville Thomson in The Atlantic, p. 136, observed: — 

 " In fresh specimens of Euplectella aspergillum the crystal framework is covered and 

 entirely masked by a layer of grey-hroimi gelatinous matter." 



The perforated dermal membrane, which is beset with numerous, small, conical pro- 

 tuberances, extends smoothly over the much folded chamber layer, and is connected with 

 it only by the outer trabecular framework, which is much riddled by the subdermal spaces 



