504 



THE VOYAGE OF H.M.S. CHALLENGER. 



by all the known representatives of the group, to conclude that the jjrimitive ancestors 

 must have had a similar structure, though in very simple form. 



As I have already noted, the generally sack-like, extremely loose body (consists 

 essentially of two approximately parallel bounding lamellte, the dermal and the gastrnl 

 membranes, l)etween which there extends the variously sinuous memlu-ana reticularis, 

 usually forming a folded chamber-layer, and supported by a framework of fine Ijeams. 

 The principal strands of this trabecular framework enclosing the parenchymal skeleton 



extend at right angles from each of the two limiting 

 lamellae, and usually meet one another in such a. 

 way that they form beams penetrating the body- 

 wall transversely, while they are also laterally con- 

 nected by numerous trabeculse, which, running in 

 another direction, form a somewhat irregular frame- 

 work, though longitudinal and transverse strands 

 predominate. 



It thus appears to me evident that, in these 

 circumstances, no more advantageous form of spicule 

 for the support of such a simple, loose Hexactinellid 

 l)ody, could be devised than the regular hexacts, 

 disposed in such a way that one radial ray unites 

 the two bounding lamellae, while the second is 



tangential, and the third longitudinal (fig. 13), just 

 as they do indeed occur in the simplest Lyssacina, 

 Ilolascus, Bathydorus, &c. 



By the firm union of all such hexacts a lattice- 

 work is formed such as we find to be developed with 

 almost ideal regularity in the younger portions of 

 Farrea. As the wall becomes thicker new layers 

 of similar regular hexacts are laid down, as we find 

 best developed among the Lyssacina in Holascus Jihxdatus, and among the Dictyonina 

 in the older portions of i^arrea-stocks, but more or less distinctly in all Dictyonina and 

 most Lyssacina. 



Thus it appears that a consideration of the mechanical conditions of the soft parts 

 to be supported affords in this case also an insight as to the utility of the specific 

 structure of the skeletal elements as here represented by the regular hexacts. 



Flo. 13. — Arrangement of Hexacts between 

 the chambers. 



