396 ECHINAXIA AND RHABDOSIGMA, 



replacing them; and at tlie same time, also, there enter into the 

 formation of the fibre, long, slender, smooth styli, which are 

 directed at a greater or less angle of inclination to the fibre, pro- 

 jecting therefrom in plumose fashion. With the advent of the 

 last-mentioned spicules, the others rapidly diminish in number 

 and soon cease. Usually the total length of fibre from which 

 the long styli project is relatively so short that they may appear 

 to form scarcely more than a divergent tuft at the extremity of 

 the fibre; occasionally, however, one or a few such spicules are 

 to be found projecting from other parts of the fibre also. The 

 terminal spicules project almost their whole length beyond the 

 dermal membrane. These long styli of the fibres are generally 

 not distinguishable from those occurring singly dispersed in the 

 axial skeleton; but in some portions of the skeleton examined, 

 taken from near the base of the sponge, a large proportion of 

 them were found to be much stouter and relatively shorter. The 

 mode of origin of the extra-axial fibres from the axial skeleton 

 is most unusual; from the longitudinal spicule-strands of the axial 

 skeleton, short lateral branches arise, which, several together, 

 converge and become confluent. As already indicated, the 

 extra-axial fibres are entirely unconnected by transverse fibres. 



Spicules.— (i.) The small smooth styli (Text-fig. 1, a) are of 

 nearly uniform diameter throughout their length to within a 

 short distance of their apical extremit}', tapering thence gradu- 

 ally to a sharp point, and are, without exception, more or less 

 curved, often rather strongly so, with the point of maximum 

 curvature situated not far below their centre. They range from 

 75 to 150/z in length and up to 65 fx in diameter: but individuals 

 below 90/jt in length or above 5/x in stoutness are extremely rare 

 except in the older portions of the sponge. Of frequent occur- 

 ence among the styli are modifications thereof in the form of 

 oxea; these are symmetrically curved, and slightly fusiform. 

 Transitional forms between the styli and oxea are rare or absent. 



(ii.) The acanthostyli (Text-tig. 1, 6) are likewise invariably 

 curved, but have the curvature restricted to their basal moiety, 

 or often even to their basal third, the remainder of the shaft 

 being straight and gradually tapered, i.e., somewhat conical; the 



