MATERIALS FOR A MONOGRAPH OF THE ASCONS. 559 



characteristic arrangement of the triradiate systems was brought 

 about which we may still observe in the olynthus of any cal- 

 careous sponge, or in the oscular tube of the Ascons (Fig. B). 



^ ^...^ 



"'^x';^ ^'...^ ^...^ ^,'. 



SP. < 



lv.\>.->-:(>.i 



'■W.V-'-- 



Fig. B.— Diagram showing the hypothetical origin of triradiate systems, each 

 from three monaxon spicules (sp.) lying between the pores (p.) 



If it be asked why only three sclerites should have fused 

 together, and not two or four or any other number, it is impos- 

 sible to say more than that the triradiate arrangement represents 

 a compromise between too great flexibility on the one hand, 

 and too great brittleness on the other. Such a question would, 

 in fact, be as diflBcult to answer off-hand as to say why insects 

 have only six legs, or mammals seven cervical vertebrae. We 

 can see that a skeleton of monaxon sclerites would afford little 

 support to an erect oscular tube, while the union of many such 

 sclerites into a lattice-work would result, in the case of objects 

 of the size of a calcareous sponge, in an excessively fragile 

 structure. 



The theory that the triradiate systems arose in this way, as 

 an adaptation to the structure of the body-wall, seems to me to 

 explain not only why they should be formed, as we see they 

 are formed, of three simple monaxons, but also their extra- 

 ordinary and striking regularity. It is the old problem of the 



