68 CLARK: NERVOUS SYSTEM OF CRINOTDS 



In its position in reference to the brachials beyond it, therefore, 

 every axillary is essentially a dissociated radial ; furthermore, in 

 their proportionate size and shape the axillaries are correlated with 

 the radials and not with any of the ossicles of the post-radial 

 series; this is well brought out in such forms as Arachnocrinus 

 bulbosus. There is thus very gccd reason for believing, aside 

 from the direct evidence available, that the chiasma within the 

 axillaries is in reality a dissociated reduplication of the conditions 

 found within each of the five interradial divisions of the calyx. 



The so-called ambulacral nervous system of the crinoids is 

 entirely different from the dorsal nervous system; it is composed 

 of extensions of or branches from the circumoral nerve ring which 

 run cut under the ambulacral grooves; these ambulacral grooves 

 are simply radial extensions from the peristomal ring which have 

 carried out with them the nervous structures subjacent to that 

 ring. The circumoral nerve ring and the ambulacral nerves 

 taken together are the equivalent of the supracesophageal gan- 

 glion in the annelid or in the arthropod. 



The chief interest in the interpretation of the crinoidal nervous 

 system as composed of five primary interradial nerves and their 

 derivatives lies in the relation which it suggests between the 

 radial and the interradial structures. 



From a study of the articulations in the crinoid arm (cf. Ameri- 

 can Journal of Science, vol. 29, p. 40; American Naturalist, vol. 

 43, p. 577) I was forced to the conclusion that the monoserial 

 arm represents the most specialized type, being derived from the 

 biserial by the slipping inward of the two rows of ossicles so that 

 they come to form but a single row, and further that originally 

 the crinoid arms were represented by ten short outgrowths which 

 became laterally united radially ; these outgrowths I ventured to 

 suggest as the potential representatives of the ten auricles cf the 

 urchins, but developed outside of instead of within the body. 



The primary interradial position of the paired nerves greatly 

 strengthens this view; for the derivatives from the primary nerve 

 cords have become greatly divergent, and unite among themselves 

 to form radial nerve cords ; as they are most intimately connected 

 with the dorsal part of the ambulacral system it seems that we 



