SPONGES 117 



one persisting, they always meet at a multiple of a right angle, and 

 the constancy of the angles between the rays at their origin is a 

 striking feature of the triaxon spicule, though often masked to 

 some extent by curvature of the rays themselves. 



In the second place, one or more of the rays of the hexactine, or 

 of one of its reduced forms, may become modified in various ways ; 

 as, for instance, by becoming curved, or by the acquisition of spines, 

 knobs, hooks, and so forth, or finally, by the development of 

 secondary branches, which in their turn may be curved or orna- 

 mented in various ways. Specially noteworthy, and often of 

 systematic importance, are the various ways in which the rays, or 

 their secondary branches may terminate. Thus to take the hex- 

 actine as an example, its rays may end in sharp points (oxyhex- 

 actine), or in knobs (tylhexactine), or discs (discohexadine). 



By the combination of modifications along different lines, there 

 results a great variety of forms of the triaxon spicule, some of which 

 have received special names and are characteristic of particular 

 families, or subdivisions of the group. 



FIG. 82. 



Characteristic Hexactinellid spicules. a, uncinate ; 6, clavula ; c, scopula. (After F. E. 

 Schulze.) 



As instances of such forms maybe mentioned the pinuli (Fig. 81, 6, c), 

 spicules usually pentactinal, sometimes, however, hexactinal, in which 

 one ray directed radially, as regards the sponge body, and always pro- 

 jecting freely from a surface, either internally or externally, develops 

 numerous small spines, and resembles a fir tree ; the various forms of 

 aster or rosette (hexaster), produced by branching of the rays, and giving 

 rise in their turn to a large series of varieties (oxyhexaster, discohexaster, 

 " floricome," " plumicome," etc., Fig. 48, o, t, Fig. 77, /.c) ; the amphidiscs 

 (Fig. 81, rf) characteristic of the Hyalonematidae, rhabdi which bear at 

 their distal extremities disc-like expansions curved towards the centre and 

 prolonged into several tooth-like protuberances ; the peculiarly ornamented 

 rhabdi known as uncinates (Fig. 82, a) and scapulae (Fig. 82, c), and the 

 monactinal clavulae (Fig. 82, b}, and many other forms too numerous to 

 mention. 



Many of the forms of the triaxon spicule depart widely in 

 appearance from the primitive type, and are often difficult to 

 recognise as belonging to it. In tracing the affinities of the 

 spicule, the axial canal affords in many instances a safe clue for 

 the detection both of those parts which are of secondary origin, 

 and those which have been lost, since, on the one hand, it is 

 not continued into the various spines or branches which may be 



