the Skeletal Parts in Echinoderms. 233 



Zar/a-lavva and in the case of many Synaptidte persist 

 thvongliout life, but also, according to the discovery of Joh. 

 Mliller, which was confirmed by Ludwig for the Asteriadse, 

 by Seraon for Ophiuridge, and by Theel for Echinidee, form 

 the basis for the construction of the spines. Nevertlieless 

 it may appear to be open to question whether we are justified 

 in homologizing with tlie wheels of the Auricularia the basal 

 wheels of the spines, which are stated by Ludwig to arise 

 from a triradiate body, and according to Thdel's latest result 

 again exhibit a tetrahedron as their starting-point. For, 

 according to ray own observations, the mode of formation of 

 the larval calcareous wheels is so peculiar that it by no means 

 allows itself to be forced into the scheme, which was con- 

 structed on the basis of our previous knowledge of the 

 building-up of the calcareous skeletal parts from mesoderm 

 cells. 



As material for investigation I made use of the splendid 

 Auricularioi which I caught in different stages o£ develop- 

 ment at the Canary Islands in the winter of 1888. As I 

 shall describe the larvaj, which attained a length of 7 millim., 

 in another place, I here dispense with an account of the 

 complicated course of their ciliated bands and of their internal 

 structure. Let it merely be remarked that the calcareous 

 wheels appear relatively very late, but then accumulate in 

 unusual abundance in the aboral tuft-shaped outgrowths of 

 the lateral regions, further on along the entire dorsal surface, 

 and much more scantily upon the ventral side. In order to 

 check ray observations the Mediterranean Auricularke were 

 also examined, which, so far as regards the formation of the 

 calcareous wheels, exhibit almost identical conditions. 



At the time of the appearance of the first calcareous wheels 

 the cellular elements of the gelatinous substance are sharply 

 differentiated into skeletogenous and connective-tissue cells. 

 The latter possess several long processes, which are much 

 ramified and are interwoven almost after the manner of felt ; 

 the skeletogenous cells, on the contrary, are spherical and 

 surrounded by a distinct membrane, in consequence of which 

 they emit no pseudopodia. The sharp histological differen- 

 tiation of the mesoderra cells, which was certainly preceded 

 by an indifferent stage, raay be essentially due to the fact 

 that the calcareous bodies originate at a remarkably late 

 period in comparison with what is found to be the case in 

 other Echinoderm larvae. The skeletogenous cells accumu- 

 late around the stone-canal and close beneath the ectodermal 

 pavement epithelium. The latter with its nuclei is always 

 distinctly discernible, even in Auricularice of the largest size, 



