MONOGRAPH OF THE EXISTING CRINOIDS. 135 



In the crustacean cuticle \vc find, in connection with the chitin, more or less 

 extensive deposits of calcium carbonate, and it is of this substance that the skeletons, 

 originally and at first solely external, of the echinodenns are composed. Although 

 the skeleton of the echinodenns as we know them to-day in a broad morphological 

 way most nearly resembles the skeleton of certain sponges and alcyonarians, the 

 ultimate origin of the echinodermal skeleton, as shown by the reduction of the 



FIG. 82. DORSAL VIEW OF THE CENTRAL STRUCTURES AND OF A SINGLE POST-RADIAL SERIES OF A SPECIMEN OF OIMANTIH-S 

 ANNULATA FROM TORRES STRAITS, SHOWING THE RELATIVE PROPORTIONS OF TUE VARIOUS PARTS. 



echinodermal skeleton to the lowest possible terms, was radically different from the 

 ultimate origin of the skeleton in these groups. At first the echinodermal skeleton 

 was a purely superficial body covering consisting of minute calcareous elements, 

 strictly homologous with, and exactly resembling, the calcified port ion of the dermal 

 investment of the crustaceans. Coincident with the evolution of the radially sym- 

 metrical echinodenns from the bilateral primitive crustacean stock was the assump- 



