568 E. A. MINCHIN. 



part I find a great difficulty in explaining some facts by natural 

 selection, but I am none the less of opinion that, in other 

 cases, it is the only possible explanation. 



Summary. 



1. The first appearance of a calcareous spicule or spicular 

 element, both ancestrally and in the actual development, was 

 probably a minute vacuole in a cell of the dermal layer, filled 

 with an organic substance perhaps identical with the inter- 

 cellular ground substance, within which the minute sclerite 

 appeared as a crystal or concretion. 



2. The ancestral sclerite, though crystalline in structure, 

 soon assumed a non-crystalline form as a whole, as an adapta- 

 tion to its secondarily acquired function of support, and as it 

 grew in size the contents of the vacuole formed the spicule 

 sheath. 



3. The ancestral form of spicule in the Calcarea was a 

 simple monaxon, placed tangentially and completely embedded 

 in the body-wall, lying between two adjacent pores. 



4. From this ancestral spicule the forms of spicule now 

 occurring in the Calcarea arose as follows : {a) the primitive 

 monaxon acquired a distal portion projecting from the surface, 

 as in the existing primary mo n axons; [b) groups consisting 

 each of three primitive monaxons became united by their con- 

 tiguous ends to form a single triradiate system; (c) to some 

 of the triradiate systems thus formed a fourth ray was added, 

 secreted by the pore-cell, giving rise to the quadriradiate 

 system ; {d) some of the triradiate systems, by loss of one ray 

 and placing of the other two in a straight line, or by loss of 

 two rays, perhaps became modified into secondary monaxon 

 spicules. 



5. The power of secreting a monaxon sclerite was primi- 

 tively possessed by every cell of the dermal layer, and this 

 condition appears to be retained in Leucosolenia. In 

 Clathrina, on the other hand, all the skeletogenous cells 

 migrate inwards from the dermal epithelium, and form a con- 



