30 THE YOYAGE OF H.M.S. CHALLENGER. 



also beset with thorns and prongs, varying in form, size, and direction (PI. LVIII. fig. 6 ; 

 PI. XL fig. 2 ; Ph XXVII. fig. 13 ; PI. XVI. fig. 8). 



In certain cases, as Marshall has shown, individual thorns become greatly developed, 

 are directed obliquely inwards, and unite with the opposite thorns of other ra)"s on the 

 same hexact. This results in the remarkable formation of octahedral margins round 

 the intersections of the lattice-like framework in many fossil, and also in some living 

 Hexactinellida (Pi. CIV. fig. 3). 



The rays of most regular hexacts are straight, but curved forms often occur. The 

 curvature may be quite irregular and undulating, but it has in most cases a definite form 

 and direction, varying in diflerent species of Sponges, and in many instances 

 characteristic. The curved rays may be smooth or rough, and are sometimes also beset 

 with prickles (PL XXVII. fig. 10). When the rays run out to a pointed extremity, the 

 name " oxyhexact" may be applied. When a knob or disc-like thickening is formed at 

 the end of each ray the term " discohexact " may be conveniently used. 



In many regular hexacts secondary rays appear, either as diverging and similar 

 terminal branches on the princij)al rays, or arranged in a circle or a disc-like transverse 

 expansion of the latter, or finally they may be uniformly distributed close to one 

 another. To such hexacts, provided with secondary or terminal rays. Carter has applied 

 the term " rosette." The number of terminal rays on each principal is generally constant 

 in one and the same kind of rosette, though it may occasionally vary in different rays, 

 even on the same rosette, and on different rosettes otherwise similar. Variations in 

 the form, direction, and dimensions of the terminal rays result in numerous kinds of 

 rosettes. 



The length of the terminal rays is generally in inverse ratio to that of the basal 

 principals, which are, as a rule, cylindrical and smooth, and less frequently rough or 

 beset with prongs (PL LXXXVIII. figs. 8, 9 ; PL XCI. fig. 7). The terminal rays are 

 sometimes smooth, sometimes rough (PL XXIV. fig. 6), or covered with prongs (PL XXV. 

 fig. 6), sometimes straight, sometimes bent in different ways, but usually in such a way 

 that the planes of curvature in all the terminal rays are directed radially towards the 

 axis of the principal. If the curvature is simple, either its concavity or its convexity 

 may be directed towards the principal axis ; if it is S-like the terminal ray bulges in its 

 proximal or inner portion towards the exterior, while the distal outer portion has its 

 convexity turned towards the axis of the principal, and bends outwards like the petal of 

 a lily. The terminal rays are seldom quite cylindrical, in many cases their diameter 

 decreases towards the free ends, in other cases the converse is observed. The very end 

 may be pointed, truncated, rounded ofi", or terminated by a sharply truncated terminal 

 expansion of a knob-, cylinder-, disc-, or bell-like form. 



As to the abundant rosettes, it is convenient to distinguish in the first place those 

 forms in which the principal rays are wholly or partly divided into two or more uniformly 



