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



since 0. Schmidt has discovered and figured such an axial canal cross, just in the middle 

 of four teeth on the little anchor spicules (of the same genus Farrea), which belong to 

 the same system as the " ClamdaB."^ 



Though I agree on this point with 0. Schmidt, I cannot accept his opinion that the 

 monact umbel and anchor spicules of Farrea are homologous with the terminal rays 

 of certain discoliexasters to which they have some resemblance. These tenninal rays 

 I regard simply as prongs without axial canals ; the umbel and anchor spicules, on the 

 other hand, I regard as true monacts. 



Mode of Union of the Spicules. 



After this general review of the most important forms of spicules, I pass to describe 

 their modes of union. In many Hexactinellida there is no distinct union between the 

 individual spicules, which either lie c|uite isolated in the soft tissue, or exhibit only a 

 slight connection or mutual support by being closely disposed side by side, or by being 

 interwoven or entangled with each other (PI. XVII. fig. 6). In other cases, however, 

 numerous spicules are bound into a firm framework by a laminated siliceous substance. 

 It may be that parallel or closely opposed rays become surrounded by a common 

 concentrically layered sheath of siliceous lamellae and so become united into a beam 

 (PI. LXXVI. fig. 5 ; PI. C. fig. 2), or it may be that the ends of the rays of one spicule 

 are opposed to the intersection nodes of another, and become fixed as if soldered ; or 

 further, it may be that the rays of adjoining spicules crossed in any dii-ection are bound 

 together by lamellae of silex, so laid down that the interspaces are filled by web-like 

 layers of siliceous substance (PL XX. fig. 3). Smaller hexacts frequently occur in which 

 the end of one ray is soldered transversely to a larger beam (PI. XXVII. fig. 8). 

 If there be no immediate contact of the . adjoining spicules, then boss or cone-like 

 elevations may grow out from the sides of two opposite beams, meet one another, and, 

 becoming surrounded with laminated siliceous material, form transverse bridges or 

 Synapticula, — which have not, of course, axial canals (PI. XX. figs. 2-4). Finally, in 

 certain conditions, both adjoining spicules, and the branches of one and the same spicule, 

 may become connected by a fine delicate lattice-work, with quadrate or rounded meshes. 

 The beams of the network arise at right angles to the lateral borders of the spicular 

 rays, and always lie in the same plane, while they are further united by transverse 

 anastomoses. Here again there is no axial canal (PI. LXIV. fig. 3). When this lattice- 

 work is greatly developed, the gaps may be quite filled up, so that finally a thin siliceous 

 plate results. It is remarkable that such lattice-work and plates are found only in 

 bounding surfaces which come into contact with solid bodies, especially where the Sponge 

 has grown on a solid substratum, Ijut also round about foreign bodies which have 



1 Spongien des Meerbusens von Mexiko, ii. p. 38, Taf. v. fig. 9. 



