THE ECHINODERMS AS ABERRANT ARTHROPODS 

 By AUSTIN H. CLARK 



CONTENTS 



PAGE 



Preface i 



The dominant characteristics of the echinoderms 2 



The larvce of the echinoderms 2 



The change from mid-somatic to inter-somatic development in the crinoids. 2 



The nervous system of the echinoderms 3 



The echinoderm coelome 5 



The water vascular system 6 



The echinoderm vascular system 7 



The echinoderm skeleton 8 



Autotomy in the crinoids 9 



The appendages of the crinoids g 



Characters of a h}'pothetical crinoid with mid-somatic development only. 10 



The crinoids and the barnacles .10 



The crinoids and the aberrant barnacles 11 



The crinoids and the starfishes 14 



The sea-urchins and the brittle-stars 17 



The feeding habits of the echinoderms and of the crustaceans 17 



Professor Patten's interpretation of the affinities of the echinoderms.... 18 



PREFACE 



Of all the larger animal groups there is none which has been the 

 subject of such diversified opinion regarding its affinities and sys- 

 tematic position as the Echinodermata. Originally assumed to be 

 related to the coelenterates on account of their radial symmetry, the 

 echinoderms were later placed near the flat-worms, the annelids and 

 the chordates. At present they are regarded as representing a very 

 distinct and isolated group, some considering them as allied to the 

 chordates, while others emphasize their points of similarity with the 

 annelids ; most zoologists, however, are reluctant to commit them- 

 selves regarding their probable affinities. 



For many years I have been convinced that the echinoderms are 

 by no means such anomalous creatures as they appear to be, and that, 

 in spite of their many and extraordinary peculiarities, they are un- 

 doubtedly closely allied to the crustaceans, and especiJilly to the bar- 

 nacles. In the following pages the reasons for this belief are given. 



Smithsonian Miscellaneous Collections, Vol. 72, No. 11 



