NO. II ECHINODERMS AS ABERRANT ARTHROPODS CLARK 9 



body organs, being on the contrary quite like the relations between 

 the chitinous or more or less calcified skeleton of the arthropods 

 and their enclosed body. The crinoid skeleton is a superficial 

 (though not external) skeleton enclosing the body and giving off 

 articulated appendages ; it therefore resembles the skeleton of the 

 arthropods more closely than it does that of any other animals. The 

 fact that it is mesodermal and calcareous seems to me to be, in view 

 of its development in every way like an ectodermal chitinous exo- 

 skeleton, and especially in view of the fact that it lies outside of the 

 ventral nervous system which runs along or just within its inner 

 surface, of purely secondary significance. 



Crinoids are undoubtedly descended from animals with an ar- 

 ticulated exoskeleton, and their articulated superficial skeleton is 

 calcareous instead of chitinous as would be expected solely because 

 of the disintegration of their ectoderm in the young stages. 



In the arthropods we find in the sessile barnacles the beginnings 

 of a transition from a chitinous exoskeleton to a calcareous mesoder- 

 mal superficial skeleton, and from the conditions in these animals it 

 is not difficult to supply the connection between the crustacean and 

 the crinoid skeleton. 



AUTOTOMY IN THE CRINOIDS 



The crinoids with more than ten arms increase the number of their 

 arms by breaking off the larval arms at the base, the stump forming 

 an axillary from which two or more arms arise. This is primarily 

 due to the inability of the brachial skeleton, a rigid calcareous invest- 

 ment of the dorsal and dorsolateral portions of the arm, to keep pace 

 with the other brachial structures in development, and is therefore 

 distantly comparable to the moulting so characteristic of the 

 arthropods. 



THE APPENDAGES OF THE CRINOIDS 



The appendages in the crinoids are of two kinds. From the base, 

 and always in connection with the chambered organ and the central 

 nen'e mass of the apical nervous system, arise uniserial jointed ap- 

 pendages ending in a strong hook. These are especially developed 

 in crinoids unprovided with a stem, and serve both as tactile organs 

 and for attachment. In their position as anterior organs and in 

 their function as tactile and grasping organs, as well as in their uni- 

 serial structure, thev recall the antennse of the barnacles. 



