70 THE PRESIDENT'S ADDRESS. 



Tetractinellida and Monaxonellida have doubtless been derived, 

 we still meet with some of the earliest stages of spicule 

 evolution. Take, for example, Dercitopsis ceylonica, collected 

 by Prof. Herdman in Ceylon, and described in my report on 

 the Ceylon sponges. Here we find the tetraxon spicule in all its 

 primitive simplicity (fig. 25), but associated with it we get 

 numerous diact spicules (figs. 26a 26c), evidently derived from 

 the tetract by loss of two of the original rays, and clearly 

 showing, by a swelling or an angulation in the middle, that 

 two rays still remain. From such obvious diactine spicules as 

 these, transitional forms lead the way to the comparatively 

 large, straight oxeote spicules which occur in the same and in 

 many other sponges, and which no longer show any trace of 

 their tetraxon and tetract ancestry. 



In Dercitopsis and its relations i.e. in the Homosclerophora 

 although there may be great differences as regards the size 

 of the various spicules, yet we cannot, as in most of the higher 

 groups, sort these spicules out into two distinct categories 

 megascleres and microscleres for innumerable gradations exist 

 between large and small. 



In the course of further evolution, however, the distinction 

 between megascleres, or skeleton spicules, and microscleres, or 

 flesh spicules, becomes very strongly marked. Both have 

 doubtless had a common origin in the ancestral tetraxon 

 archetype, but whereas the former are obviously adapted as the 

 principal skeletal elements, and are arranged accordingly in 

 the sponge, the latter are usually scattered at random through 

 the soft ground substance like plums in a pudding, and neither 

 in form nor arrangement show any evident adaptation to the 

 requirements of the organism. 



Indeed, the microscleres are usually so extremely minute, 

 requiring high powers of the microscope to make out their 

 true form, that is impossible to believe that their presence can 

 exercise any important influence upon the well-being of the 

 sponge. Still less is it possible to believe that the particular 

 shape which they may assume, which is often highly remarkable, 

 can be of any consequence to their possessor. There are, of course, 

 exceptions to this, as to every generalisation, and sometimes we 

 find microscleres forming a dense protective external crust, as 

 in the case of the " sterrasters " of Geodia, or projecting into the 



