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



On the supposition that the arrangement of the parenchymalia will be least modified 

 where the original form of the Sponge is a thin-walled sac, and the typical six-rayed 

 condition of the spicules most distinctly persist, I will start from such conditions as 

 occur in Holascus and Farvea. Here the chief supporting framework of the parenchyma 

 is formed of large regular hexacts, which are disposed at right angles or parallel to the 

 bounding surfaces. These forms, which may be termed directalia, simply lie in the one 

 case with their corresponding rays apposed to one another (PL XVI. fig. 2 ; PI. XVII. 

 fig. 2), while in the other they are closely united into a rigid framework (PL LXXIII. 

 fig. 2 ; PL LXXVL fig. 5). 



The invariably six-rayed sjaicules, which are concerned in the formation of the 

 continuous dictyonal-skeleton, and which I call dictyonalia, are always distinctly 

 recognisable as such, for even when they have not become connected into a regular cubical 

 meshwork, but only in an irregular manner, they are readily recognised by their axial canals. 

 This is not the case with the spicules indicated as principalia, which form the chief 

 supporting framework of the parenchyma in the Lyssacina, for these, like the dictyonalia, 

 do not retain their typical position, and further, by no means always exhibit the six rays, 

 being frequently reduced to pentacts, tetracts, or even indeed to simple diacts. Thus, 

 in Euplectella aspergillum, for example, tetracts occur in the formation of the quadrate 

 lattice-work on the inner side (PL II. figs. 2, 5), while in Eup>lectella suherea (PL V. 

 fig. 15) and in Euplectella nodosa (PL XIV. fig. 2) pentacts occur for the same purpose 

 and in the same positions ; in Crateromoipha, Rhahdocalyiitus, and Aidochone, 

 numerous strong diacts are found both in the body and in the stalk. 



Closely apposed to the strong rays of the principalia slender elongated spicules 

 frequently occur, with two or three rays, but seldom more (PL XVII. fig. 6). These I 

 would call comitalia. They are sometimes straight and parallel to the corresponding 

 ray of the principal spicule, but they usually extend in a winding and wave-like manner 

 on or round the outer surface of the principal. 



In regard to the position of the parenchymalia, which are found more or less 

 abundantly between the dictyonalia or the principalia and their attendant comitalia, 

 few general points can be noted. While elongated spicules of this category frequently 

 run at right angles to the surface (as the imcinata of many Dictyonina), or are disposed 

 in any other way with complete regularity, very numerous, small, crowded spicules 

 often occur, like the small hexacts, the rosettes and their derivatives, which are almost 

 always scattered quite irregularly in the parenchyma. 



