228 Transactions of the [''jZlZXyTil\t 



basis-substance, it seems to be diffused throughout the spine, though 

 more tenacious and membrane-Hke in its external portions. But in 

 the large spines of those species which add layer to layer by exo- 

 genous growth, it seems to me not improbable that the sarcodic 

 basis-substance may after a time cease to occupy the interspace- 

 system of the older and inner portions of the spine, and that 

 it may be for the most part restricted to the newer and outer 

 layers ; just as we know that in an ordinary Exogenous stem, the 

 outer layers only of the wood take an active part in its functional 

 changes. And it is a confirmation of this idea, that such is cer- 

 tainly the case in the segments of the arms of the fully-developed 

 Aniedon ; in which I have often found the sarcodic basis-substance, 

 after the decalcification of the segment, to form a mere shell, — 

 scarcely any trace of it being discoverable in the interspace- system 

 of the central part of the calcareous reticulation. And thus, 

 although the notion of an " investing membrane " to these spines, 

 performing the functions of the periosteum of bone, proves to be 

 fallacious, we seem justified in regarding the organic basis-substance 

 of their outer last-formed layers as the part essentially concerned in 

 effecting these remarkable reproductions. The marvel is that they 

 should be able to derive the materials of their work from such a 

 remote source as the nutritive apparatus within the shell, no vessels 

 of any kind having been found to pass through its walls to convey 

 nutrient fluid to the spines borne upon its exterior. 



