1908.] SPICULES OF CALCAREOUS SPONGES. 671 



PI. XXXYII.) which are abundant and easily found, and at 

 the same time are not so inconveniently large as in G. contorta. 

 It is seen that in the distal blunt curved portion (fig. 13) the axial 

 filament is very thick, and forms a broad band showing a 

 dark double contour enclosing a central lighter portion ; in the 

 proximal straight pointed portion (fig. 14) the filament 'appears 

 as a single thread, as in the rays of the triradiates. Hence the 

 monaxons of A.falcata show a noteworthy difierence from those 

 of C. contorta, a point to which we shall i-eturn. 



In C. clathrus (fig. 15, PL XXXVI., fig. 16, PI. XXXIV., 

 and fig. 17, PI. XXXVII.) it was found that in the majority 

 of the triradiates the axial filament terminated abruptly at an 

 appreciable distance from the end of the spicule, and was not 

 continuous with the sheath. This condition is probably correlated 

 with a peculiarity in the mode of growth of the spicules of this 

 species which was pointed out by Minchin (1898), namely, that 

 the apical formative cell or "founder" does not leave the ray, 

 but persists and helps, apparently, to secrete the blunt thickened 

 termination of the ray which characterises this species. In a few 

 cases, however, a continuation of the axial filament up to the 

 sheath could be seen distinctly (fig. 16), but from the shape of 

 the rays it is probable that in such cases the spicules were not 

 quite full-grown. 



The spicules of Leucosoleniid^ and Heteroccela examined by 

 us appear, with one exception presently to be described, very 

 difierentjfrom those of Olathrinidse. The first impression derived 

 from examination of them is that no axial filament is present. 

 A more careful study reveals a filament pi-esenting a certain 

 similarity to that of the monaxons of Ascandra falcata, namely, 

 a broad band towards the base of each ray, which narrows to a 

 delicate filament towards the tip of the ray (fig. 18, PL XXXVII.). 

 It is very difficult to get satisfactory photographs of the filament, 

 both on account of its feeble staining powers, and of the difficulty 

 of getting it in focus, due to the frequent curvature of the rays. 

 In connection with the phylogenetic speculations of Minchin 

 (1900, p. 109, and 1908), it is of interest to find so great a differ- 

 ence in the structure of the spicules of Clathrinidfe on the one 

 hand, and of Leucosoleniidse and Heteroccela on the other hand. 

 It may be pointed out further that, as stated above, the studies 

 of Ebner and Biitschli, with regard to the presence of an axial 

 filament, were based entirely upon examples of the Heteroccela. 



An exception, however, to the foregoing statements concerning 

 Heteroccela is furnished by the remarkable sj)onge Heteropegma 

 nodus-gordii Folej., of which, by the kindness of Professor Dendy, 

 we have been able to examine a specimen. In this sponge we 

 find the axial filaments very distinct, especially in the triradiates 

 of moderate size *, in whicli they present the same characters as 

 in Clathrina contorta, and can be photographed easily at low 



* For figures of the spiculation of Seteropegma see PolejaefF, ' Challenger ' 

 Keports, Zool. vol. viii. pare xxiv. (1883) pi. iv. tigs. 1, a-d, 



Proc. Zool. Soc— 1908, No. XLIII. 43 



