MONOGRAPH OF THE EXISTING CRINOIDS. 325 



which forms a stout envelope all about it, being of greater thickness than the 

 diameter of the vessel itself. 



In cross section ganglion cells are seen which ai'e mostly bipolar and lie 

 either peripherally or scattered between the fibrillse ; they are arranged in definite 

 groups. 



Within each cirrus segment, as within each brachial and pinnular, four 

 nerviiles are given off from the main stem at opposite corners; these innervate 

 the ligaments between the segments and the outer layers of the segments themselves. 



BRACHIAL NERVES. 



Neri^es of the division series. 



Each half of the five primary interradial nerve stems, which divide imme- 

 diately upon leaving the central organ, joins within the calcareous substance 

 of the radial the adjacent half of the neighboring primary stem to form one 

 of the five secondary interradial nerve stems. Very shortly after this union the 

 five secondary nerve stems are connected all around the calyx by an intraradial 

 commissure, which is pentagonal in form. 



From the radials the secondary nerve stem passes as a single cord through 

 the IBr, ; but on entering the IBrj (axillary) it divides into a right and left 

 branch, these two branches running to the center of the right and left distal 

 faces of the axillary. Just before leaving the axillary the two branches are con- 

 nected by a transverse commissural band of fibrillar and by a chiasma formed by 

 two obliquely placed bands which cross one another and, joining the opposite 

 branches at the ends of the transverse commissure, furnish additional communi- 

 cation between the right and left branches. 



Every subsequent division series reduplicates the conditions found in the IBr 

 series, and every subsequent axillary contains a similar transverse commissure and 

 chiasma. 



The branches of the secondary nerve stems leave the axillaries as single cords 

 and, gradually decreasing in size distally, pass along the dorsal side of the 

 undivided arms in tubular channels perforating the brachials as the axial nerve 

 cords of the arms. 



Finer structure of the hrnehial nerves. 



The nerve cords which emanate from the central organ are throughout their 

 whole length solid and approximately circular in section. In Anfcdon mediter- 

 ranea the diameter of the primary nerve stems is 0.2 mm., that of the two deriva- 

 tives of each of these which unite to form the axial cord of the arms 0.1 mm., 

 and that of the branches forming the chiasma 0.02 mm. 



The fibrillfe of the dorsal nerves are exce])tionally fine and run everywhere 

 parallel to each other and closely appressed. The short bundles of fibrillfe which 

 form the chiasma in the IBr.^ (axillary) pass each other in the center, almost at a 

 right angle, without intermingling. 



About the periphery the nerve cords are studded with large, mostly multi- 

 polar, ganglion cells; these are abundant dorsally, forming a sort of peripheral 

 capping, and occur irregularly in the central portion. 



