66 CLAEK : NERVOUS SYSTEM OF CRINOIDS 



complex "dorsal" nervous system of the crinoid may be compared 

 with the relatively simple ventral nervous system of the annelid 

 or arthropod. 



The five primary nerve cords arising from the central nerve 

 mass are interradial in position; typically (cf. Proceedings U. S. 

 National Museum, vol. 35, p. 87, fig. 1) they very soon divide 

 into two, these two derivatives diverging and uniting with sim- 

 ilar derivatives from the adjacent primary nerve cords just within 

 the outer face of the radials; an intraradial commissure connects 

 all the points of union of the branches from the primary nerve 

 cords; five intrabasal commissures connect the two branches of 

 each of the pairs of secondary nerve cords shortly after the divi- 

 sion of the primary cords. From the radials single nerve cords 

 (which may, however, exhibit slight traces of a longitudinal 

 division), each composed of half of the two primary interradial 

 cords in the two adjacent interradii, extend outward into the 

 arms. 



Now in monocyclic crinoids all the derivatives from the central 

 nervous capsule, or from its extension into the stem, are inter- 

 radial; and it seems to me that we must regard an interradial 

 position as primitive for the nerve trunks arising from this cen- 

 tral capsule. If this view is correct, the dorsal nervous system 

 of the crinoid consists of five primary branches which divide and 

 diverge widely so that they become united by their outer sides 

 to the similar derivatives from adjacent primary branches, each 

 of the five pairs of secondary nerve cords being united by two 

 commissures, one within the basals, the other within the radials. 

 Each one of these five pairs of secondary nerve cords with the con- 

 necting commissure is strongly suggestive of the ladder-like ven- 

 tral nervous system of primitive molluscs, phyllopod crustaceans, 

 nemerteans, peripatus, etc. 



This interpretation of the complex nervous system of Isocrinus 

 or Metacrinus as primarily composed of five interradial nerve 

 pairs which have become united exteriorly is strongly suggested 

 by the nervous system of the fossil genus Encrinus as worked out 

 by Beyrich ; Encrinus has biserial arms, which are more primitive 

 than the monoserial type as seen in Isocrinus or Metacrinus, 



