MATERIALS FOR A MONOGRAPH OF THE ASCONS. 557 



them. In this way arose from a common source the two 

 primitive types of spicule which we find in the Calcarea. 



Before proceeding to discuss the factors which may be sup- 

 posed to have brought about this course of evolution, I will 

 first examine all the objections which occur to me against 

 my theory of the origin of the triradiate and quadriradiate 

 system by fusion of skeletal elements primitively distinct. 



1. A great objection, at first sight, to regarding the many 

 rayed calcareous spicules as compound bodies, is the well- 

 established fact that they behave optically as single crystals 

 (Sollas and Ebner, see above). This fact certainly seems 

 strong evidence for regarding them as simple and uniform 

 bodies. But it may be pointed out that, whatever their 

 ancestral history, each ray does actually arise from a separate 

 mother-cell in the course of the development. It is just as 

 difficult to comprehend why the united products of a number 

 of independent cells should behave as a single crystal, as to 

 believe that a body of this kind arose ancestrally from a fusion 

 of separate elements. This is especially so in the case of the 

 quadriradiate systems, where the fourth ray has an origin 

 quite distinct from the basal system, and is even formed at a 

 later period. I am justified therefore, I think, in claiming 

 that my theory of a fusion of skeletal elements to form a 

 structure which behaves as a single crystal individual, does 

 not present greater diflTiculties than do the ascertained facts of 

 development.^ 



2. It might be urged against the views here advocated, that 

 no other sponge spicules are known to arise by fusion of 

 separate elements. This is an argument which at most would 

 only excite a bias against the theory, but could not afford a 

 positive disproof of it. It is certainly true that, so far as is 

 known, the polyaxon spicules of other sponges arise always 

 each in a single cell. On the other hand, instances of secon- 

 dary deposit of skeletal material uniting primitively distinct 

 spicules are not wanting. We may cite the case of the 

 compound desmas of the Lithistida; the secondary siliceous 



* For further remarks on this point see Addendum B (infra, p. 572). 



