Sponge Spicules and their Names. 211 



used the prefixes " holomono- " (all actines with one end- 

 spine) and " holopoly- " (all actines with more than one end- 

 spine), in contrast with " hemi- " (some actines with more 

 than one, some with only one end-spine), but the sight of the 

 word holopolydiscohexaster made me forbear! Even that 

 apparently terrible word, however, is in reality a convenient 

 shorthand term. 



Monoxyhexasters often bear a deceptive resemblance to 

 hexactins, but the presence of the coreless end-spine betrays 

 the real nature. " Das also war des Pulels Kern " ! 



Ijima was the first to suggest what seems to me the logical 

 terminology for spicules with one end-spine to each actine. 



It is my conviction that the spicules of Corbitella speciosa 

 (Q. & Gr.) and Ileterotella corbicula, Bowbk., which Ijima 

 terms spiny microhexactins will certainly be found to be 

 monoxyhexasters (see Contrib. ii. pi. figs. 3, 9, 20-23) *. 



Monoxyhexasters are found only in the suborder Hexastero- 

 phora, and microhexactins are never found in that suborder. 



(4) Microhexactins are only found in the Amphidisco- 

 phora. Often these spicules have a granular surface excepting 

 at the smooth centre, where alone the axial canals can be 

 seen, being invisible in the rest of the length, for the same 

 reason that it is impossible to see below the ruffled surface of 

 a lake. It is nearly always possible to trace the canals, 

 however, in other smoother examples from the same specimen. 



(5) Hexadisk is not an entirely satisfactory term, because 

 most calycocomes are also, in a sense, hexadisks. In the 

 Amphidiscophoran hexadisks the end-spines are centripetal, 

 but in the calycocomes they are directed centrifugally away 

 from the disk or capitulum. The essential morphological 

 difference is solely the direction of the end-spines, and the 

 term that expressed that essential feature would have been 

 the most suitable. For, according to my definition, a hexa- 

 disk as usually understood is undoubtedly a hexaster with 

 centiipetal end-spines (see PI. VIII. fig. 9, showing the actine 

 of an amphidisk with the end-spines). 



I have put forward the theory that the difference between 

 amphidisks and hexasteis is due to differences in centripetal 

 pressure at an early stage in the history of the Hexactinellida 

 (Ann. & Mag. Nat. Hist. (8) iv. 1909, p. 479). The form 

 of the amphidisk and hexaster is the index of the resultant 



* Since the above was written I have managed to remove the window- 

 pane cover slip from a valuable slide of H. corbicida in the Bowerbank 

 Collection and to view the spicules under oil-immersion. I find, as I 

 anticipated, that the so-called spiny microhexactins are spiny monoxy- 

 hexasters. 



