208 Mr. R. Kirkpatrick on Ilexactinellid 



XXXI. — On Ilexactinellid Sponge Spicules and their Names. 

 By R. Kirkpatrick. 



(Published by permission of the Trustees of the British Museum.) 



[Plate VIII.] 



IIexactinellid spicules are often divided into two classes, 

 megascleres and microscleres. The former uphold, fix, and 

 protect the body as a whole, and the latter support the strands 

 of the trabecular network. 



The spicules may be classified in another way and on a 

 purely morphological basis. Let it be supposed that examples 

 of all known recent Hexactinellida have been put into a huge 

 cauldron and boiled up with nitric acid, and the residual 

 spicules arranged into groups according to their morphological 

 characters, the sorter being biased by no phylogenetic fancies 

 or speculations whatsoever. It would be found that the 

 immense multitude of spicules would fall into two great main 

 groups : let us call them for the moment A and B. In 

 group A all the rays would be hollow (i. e. with axial canal) 

 throughout, but in group B only the central end would be 

 hollow and the rest of the ray or ray-system solid. 



Definition. — The part of the ray with the axial canal is 

 here termed the " actine," the solid part or parts of the so- 

 called ray or ray-system being termed end-spines or " distal 

 appendages." 



In describing specimens I shall continue to use the terms 

 main or primary and end or secondary " ray " ; but here it is 

 necessary to emphasize the fundamental difference between 

 the hollow and solid parts found in the rays of spicules 

 belonging to group B. The use of the term end ray, as con- 

 trasted with main ray, is not, however, quite logical, because 

 the difference is not merely one of position, like, say, the 

 stem and branch of a tree, but there is a fundamental 

 difference of character, the main ray being hollow and the 

 so-called end ray solid. 



What names should be given to groups A and B? Hom- 

 axial or homactinal and heteraxial or heteractinal at first 

 suggested themselves; but I soon found that these terms 

 were wholly illogical. For both groups of spicules, i. e. all the 

 spicules, are homaxial and homactinal, but group B has distal 

 appendages or end-spines at the ends of its actines. 



The names hexactin, hexaster, aster, are already in use, and 

 the two former could not well be made to include pentactins, 



