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the sphæridiæ are always placed in the ambulacral areas, though 

 otherwise a great variation prevails as to their position and 

 number; in the Asterids they are placed interradially. This 

 fact, to be sure, seems to be a serious objection, but, in my 

 opinion, it does not disprove the homology of the mentioned 

 organs. The radial or interradial position is an extremely im- 

 portant character in Echinoderm morphology, and as a whole 

 it must be maintained that the relation to the radia or interradia 

 is a criterion of homology. In this case, however, it cannot be 

 applied. It must be remembered that the sphæridiæ are only 

 transformed spines; but spines are not exclusively radial or 

 interradial, and nobody, I think, would maintain that the ambu- 

 lacral (radial) spines of Echinids are not homologous to the 

 interradial spines of Asterids. Further it must be remembered 

 that the homology of the ambulacral plates in Echinids is very 

 uncertain; at any rate they are scarcely homologous to the 

 ambulacral plates of Asterids, more likely they are homologous 

 with the adambulacral ones of the Asterids. In that case the sphæ- 

 ridiæ in both Asterids and Echinids would be placed on plates 

 of the same morphological value. But, be that as it will, at 

 any rate it seems to me undoubtful that the mentioned struc- 

 tures in the Asterids are true sphæridiæ. — A careful examina- 

 tion of their histological structure especially of the thick cove- 

 ring of skin is very desirable. 



In itself it is not so very surprising to find sphæridiæ in 

 Asterids. Pedicellariæ are likewise found only in Asterids and 

 Echinids (with the single exception of Trichaster, in which 

 Ludwig has found a curious kind of pedicellariæ). But it is 

 of no small importance that the Homologa of these curious 

 organs, hitherto thought characteristic of Echinids alone, have 

 thus been found also in Asterids. In accordance with the well 

 known fact that pedicellariæ are found only in certain families 

 of Asterids the sphæridiæ are likewise not found in all Aste- 

 rids, but are known to occur only in the families Pterasteridœ 



