730 



ON CONCRETIONS, SPICULES, ETC. 



[CH. 



seems due to a failure to recognise this twofold or alternative 

 symmetry. 



In all these latter cases it is the arrangement of the axial rods — 

 the "polar symmetry" of the entire organism — which hes at "the 

 root of the matter; and which, if only we could account for it, 

 would make it comparatively easy to explain the superficial con- 

 figuration. But there are no obvious mechanical forces by which 

 we can so explain this peculiar polarity. This at least is evident, 

 that it arises in the central mass of protoplasm, which is the essential 

 living portion of the organism as distinguished from that frothy 



Fig. 343. Acanthometra sp. A 

 derivative of the Dorataspis 

 figure. After Mielck. 



Fig, 344. Phractaapis prototypus Hkl. 



peripheral mass whose structure has helped us to explain so many 

 phenomena of the superficial or external skeleton. To say that the 

 arrangement depends upon a specific polarisation of the cell is 

 merely to refer the problem to other terms, and to set it aside for 

 future solution. But it is possible that we may learn something 

 about the fines in which to seek for such a solution by considering 

 the cage of Lehmann's "fluid crystals," and the light which they 

 throw upon the phenomena of molecular aggregation. 



The phenomenon of "fluid crystallisation" is found in a number 

 of chemical bodies; it is exhibited at a specific temperature for each 

 substance; and it would seem to be limited to bodies in which 

 there is an elongated, or "long-chain" arrangement of the atoms 

 in the molecule. Such bodies, at the appropriate temperature, tend 

 to aggregate themselves into masses, which are sometimes spherical 



