558 E. A. MINCHIN. 



deposits uniting the spicules in the Dictyonina ; and finally 

 the secondary deposits of calcite uniting the calcareous spicules 

 in the Pharetrones, of which Doderlein has so recently (1897) 

 described a living example in his Petrostoma Schulzei. 

 These examples are sufficient to show, were evidence required, 

 that spicules may become united together secondarily to form 

 compound structures. 



3. A quite trivial difference, as it seems to me, can be pointed 

 out between the formation of a ray of a triradiate, and of a 

 primary monaxon spicule. While in the former the actinoblast 

 divides, in the latter, so far as is known, the mother-cell 

 remains single. But the history of the gastral ray shows 

 plainly that it depends on the size of the spicule to be formed, 

 whether or no the mother-cell remains undivided. And for 

 my part I much doubt whether, in the case of primary mon- 

 axons which attain a large size, the secreting cell always 

 remains single. 



None of these objections are in any way a serious obstacle, 

 it seems to me, to regarding the triradiate and quadriradiate 

 systems not as true or primary spicules, but as compound 

 spicular systems. To my mind it is essential to the definition 

 of a true spicule or primary spicular element that it should be 

 formed by a single mother-cell, or by the descendants of one 

 such cell. In this way each ray of a triradiate system repre- 

 sents a single spicule, homologous with a primary monaxon, 

 and the whole structure is a compound or secondary spicular 

 system. Examples of primary siliceous spicules are tlie ordi- 

 nary monaxons and tetraxons which arise each in a single cell, 

 while the desmas of Lithistida are instances of secondary 

 systems. 



This view of the origin of the triradiate systems completes, 

 as it seems to me, and extends Schulze^s theory that they arose 

 as an adaptation to the structure of the body-wall of the 

 simplest Calcarea. The primitive monaxon sclerites lying in 

 the body-wall would soon take on an arrangement suited to its 

 structure, and came to lie probably one between every pair of 

 adjacent pores. Then, by union of the monaxons in trios, the 



