Spines of Echinoidea. By Prof. F. Jeffrey Bell 849 



be due to the increase in the thickness of the crust, but our observa- 

 tions show that for Cidaris metularia (a) while one-fourth of the 

 diameter of the base is crust in the smaller, only one-fifth is 

 crust in the larger specimen, and (/3) that of the tip two-fifths is 

 crust in the smaller, but only one-third in the larger specimen. 

 The growth, therefore, is due more to the internal portion thanto 

 the external crust, and, as we obtain the same kind of result with 

 the base as with the tip of the spine, we cannot say that the crust 

 is as thin as it is because it has been worn away, or, at_ any 

 rate, make that more than a very subsidiary reason for the differ- 

 ence. The varying proportions presented by PliyUacanthus ofier 

 some difficulties on other scores, but it is abundantly clear that the 

 crust does not become proportionately thicker than the interior. 



2. The evidence now collected as to the power of growth of the 

 spine as a whole, is supported by the structural characters of the 

 tips of certain spines which are remarkable for their form. One 

 of these, which is perhaps the most striking, is that of Goniocidaris 

 jiorigera, which are widened out at their free ends and provided 

 along their sides with spinose projections in the way that is shown 

 in pi. XIII. fig. 1. A transverse section of such a spine (fig. 2) 

 shows that the outgrowths have a comparatively thin crust, and 

 that the greater part is formed by exactly the same kind of 

 reticular tissue as that which occupies the greater part of the 

 interior of the spine. Here, at any rate, there is a relation between 

 the characters of the surface of the spine, and the disposition of 

 the tissue covered by the ostracum. 



Goniocidaris geranoides, a figure of which has been given by 

 Prof. Mackintosh,* is an example of a spine in which the projec- 

 tions are again distinctly formed by the reticular tissue, and 

 not by a crust that is in any sense amorphous. ^ Phijllacanthus 

 imperialis (fig. 3) and to a less extent P. verticillatus, afford us 

 examples of spines in which the external contour of the crust is 

 distinctly seen to be dependent on the arrangement of the layer 

 within. 



3. Finally, to conclude this line of argument, we observe that in 

 all cases the ostracum is penetrated by spaces which we cannot, 

 from what we now know, look upon as being otherwise than 

 occupied during life by a protoplasmic ground-substance. The 

 continuation of the interspaces into the crust is, of course, better 

 seen in some than in other species ; fig. 4 shows the arrangement 

 which obtains in Stephanocidaris hispinosa, the structure of the 

 spine of which is now illustrated for the first time. 



When we combine the information afforded by the facts and 

 figures now offered, with that which has been acquired by pro- 



♦ Loc. cit. - 



