•_M1 



THE AMEBIC AN NATURALIST [Vol. XLIV 



ganglia became retracted into the subcesophageal ganglion, now 

 become the dorsal nerve mass. 



Thus we may very easily derive the radiating nervous system 

 of the crinoid from the bilateral nervous system of the 

 arthropod, the circumoral nerve ring of the crinoid being 

 derived from the supracesophageal ganglion of the arthropod, 

 and the dorsal nervous system from the suboesophageal ganglion. 

 In this connection it is significant that the circumoral nerve ring 

 innervates the same structures and possesses the same functions 

 as the supraoesophageal ganglion, while the dorsal nervous sys- 

 tem is identical with the subcesophageal ganglion, plus the ven- 

 tral nerve cord, in the nature of its duties. 



The assumption of the pentaradialo form by the crinoids lias 

 produced a complex condition of orientation; for the so-called 

 "ventral" surface of the crinoid, in terms of arthropod or anne- 

 lidan orientation, is equal to the anterior end, plus the posterior 

 end, and plus such part of the dorsal surface as is not shoved to 

 one side by the approximation of the two extremities of the in- 

 testinal canal; while the "dorsal" surface of the crinoid is the 

 equivalent of the entire ventral surface of the arthropod, plus 

 more or less, possibly nearly all, of the dorsal surface. 



Austin H. Clark. 



