SHORTER ARTICLES AND DISCUSSION 



Although at first si-lit the nervous system of a erinoid ap- 

 pears to be so radically different from that of an invertebrate of 

 the more usual bilateral typo that no satisfactory comparison is 

 possible between them. T believe that there is no difficulty at all 

 in deriving it from the latter. 



The nervous system of a crustacean, worm or insect consists 

 typically of a supraresophagea] -.-midion united by a pair of 

 cireuuKosophnu'eal ganglionic connectives to a more or less 

 marked subcesophageal ganglion, from which there runs back 

 along the ventral side of the animal a long nerve cord, or pair of 

 nerve cords, marked at intervals with ganglia. The anterior end 

 of the digestive tube passes between the two chief ganglionic 

 masses, as the names of all these structures indicate. 



The ancestral erinoid was bilateral, and therefore possessed a 

 nervous system constructed according to this plan. With the 



sumption by the animal' of a more compact form, just as the 

 nerve cord has been shortened in Cancer as compared with Pal- 



compared with Diapheromera, or, better, in the crustaceans as a 

 class as compared with the annelids as a class. In the crinoids 

 the shortening progressed still further; locomotion, other than 

 casual or accidental, ceased; the anterior end of the intestinal 

 canal became deflected upward and pressed upon the anterior 

 part of the suprau>sophagcal ganglion which -rave way before it 

 and became deeply crescent ic : at the same time the ventral 

 nervous cord was retracted into a short protuberance from the 

 suh.eNoplm-val ganglion. Finally the horns of the crescent 

 formed from the suprao'sophageal ganglion met in front of the 

 throat so that what was originally a ganglion mass became a 

 nerve ring, the two connectives became broken up into numerous 

 connecting fibers, and the whole ventral nerve cord with its 



