MONOGRAPH OF THE EXISTING CRINOIDS. 135 



In the crustacean cuticle we 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 ccliinodenns are composed. Although 

 the skeleton of the echinoderms 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 or the central structures and of a single post-eadul series of a spcawEN or comanthus 



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



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

 ultmiate 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 portion of the dermal 

 investment of the crustaceans. Coincident vrith the evolution of the ratlially sj-m- 

 metrical echinoderms from the bilateral prmiitive crustacean stock was the assump- 



