78 



ling to an extraordinary degree Ech. elegans, with which species 

 it has, however, nothing to do). In this specimen the spines 

 of the buccal plates were partly transformed into sphæridiæ, all 

 transitional stages being found between common spines and true 

 sphæridiæ. (Textfig. 1 — 3). — Now these secondary mouthspines 

 in Pteraster militaris are constructed just in the same way as 

 the sphæridiæ: the lower part has retained the common calca- 

 reous structure, whereas the upper part has been transformed 





Fig. 1 — 3. Transformation of spines into sphæridiæ, in Echinus esculentits. 



1. Spine of the typical form. 2. Transitional form. 3. Sphæridia, 



(Seibert: Obj. II. Ос. I.). 



into a clear, compact, glass-like mass (PI. 11. Fig. 4). That the 

 surface is not smooth, but covered with small, scalelike thorns 

 (Fig. 5) is no proof against their being true sphæridiæ; also in 

 the Echinids they are often more or less thorny. Neither is the 

 fact that they are covered with a thick skin an objection against 

 my view, the sphæridiæ of Echinids being likewise covered with 

 skin (though a thin and transparent one). In short, I cannot 

 find a single essential difference between these structures and 

 the sphæridiæ of Echinids, except in their position. In Echinids 



