REPORT ON THE HEXACTINELLIDA. 279 



ray of a hexact of the new outer layer is apposed, and both become enveloped in a 

 common concentrically laminated siliceous sheath. It is obvious that in this way the 

 older and lower portions of the tubes may gradually increase in thickness. 



The thick, firm, spongy mass, which forms the pedicel on the older stocks and the 

 flatly expanded basal plate, has had another origin. Besides the apposition of a new 

 layer of the cubical lattice-work, we have here to deal with the interposition of numerous 

 smaller hexacts in the already formed meshes. And while the external apposition of 

 new layers leads in general to the regular formation of rectangular or cubical meshes, the 

 interposition of small hexacts does not usually occur in the radial tangential direction, 

 but at very various angles to the main strands, so that an irregular narrow meshed 

 network results (PI. LXXVI. fig. 2). Where the basal plate is in direct contact with 

 the substratum, as in all other Hexactinellids fixed on a solid basis, a narrow meshed 

 bounding plate is formed, which arises mainly by the abundant development of 

 synapticula between the bands of spicules. 



While the strands of the single-laj'ered dictyonal network which lie parallel to the 

 surface of the tube are cylindrical and smooth, the intersections of the net exhibit 

 conical prominences projecting at right angles both outwards and inwards, and always 

 beset with small tubercles or rough elevations. In the many-layered dictyonal frame- 

 work, the radial beams extending between the la5'ers, which lie parallel to one another 

 and to the tube wall, exactly resemble the tangential beams in their cylindrical shape 

 and in their smooth surface, while here also the prominences projecting from the outer 

 and inner surface of the whole lattice-work are always tubercled and rough. The length 

 of these freely projecting conical prominences varies as much as their form, and that 

 between tolerably wide limits. They are generally straight or only slightly curved ; are 

 longer in the younger portions, especially in the single-layered framework, and shorter on 

 the surface of the many-layered framework of the older regions. On the surface of the 

 stalk and on its basal expansion they are either altogether absent, or represented only I33' 

 small knob-like elevations. 



At the points of intersection of the dictyonal framework there is a slight thickening 

 gradually increasing with age, but this is not in any way marked off from the strand, nor 

 different from the latter in the character of its surface. 



The composition of the whole framework from separate hexacts may be most readily 

 recognised on those specimens which have remained dead in the sea for some time and 

 are thus macerated. The axial canals of the individual hexacts have through the 

 dissolution of the loose inmost layers become more or less markedly -nidened, and are 

 thus in their disposition and extent readily observed (PI. LXXVI. fig. 5). In fresh 

 specimens, however, they can usually be detected as very fine canals (PL LXXI. fig. 

 3, &c.) 



The length of these axial canals is often surprising. While they usually extend only 



