460 COMPARATIVE ANATOMY chap. 



The chambered sinus which lies in the centrodorsal is surrounded 

 l)y a cup-shaped envelope, which consists of ganglion cells and nerve 

 fibres. The latter are arranged more or less concentricallj' around the 

 sinus. The nerve envelope of the chambered sinus is continued along 

 its prolongation, the stalk canal, and along the canals of the cirri. All 

 these canals are in fact surrounded by a nerve sheath whose fibres run 

 longitudinally. The large apical nerve trunks, which run from the 

 nerve envelope of the chambered sinus into the radii (enclosed in 

 the nerve canals), consist of nerve fibres and ganglion cells. A kind 

 of metamerism is apparent in them, since they become somewhat 

 thickened in the consecutive ossicles of the arm, and give off nerves at 

 regular intervals corresponding with these ossicles. The apical nerve 

 trunks divide with the arms, in whose nerve canals they are enclosed, 

 and run to the tips of the pinnulse. 



Round the chambered sinus, commissures are formed between the 

 nerve trunks arising from the nerve-envelope of the sinus. The courses 

 of these commissures are explained by the diagrams of the nerve canals 

 in which they are enclosed (Figs. 327-330, p.' 378). 



In Aiitedon and other forms, in the second costal, where the five 

 primary nerve trunks fork to make the ten brachial nerves, a 

 peculiar ehiasma neFVorum braehialium is formed {cf. Figs. 327 

 and 329, p. 378). The two ramifications of the nerve, which by 

 their crossing form the ehiasma, run one above the other without 

 any mingling of their fibres. Further, the two brachial nerves form- 

 ing one pair ai'e connected immediately beyond the ehiasma by means 

 of a transverse commissure. 



In each ossicle of the arm, the apical brachial nerve gives off an 

 upper (oral) and a lower (apical) pair of nerves. These nerves seem to 

 be principally sensory. They ramify richly in the calcareous substance 

 of the joints. The increasingly fine ramifications, which radiate to 

 the surface of the arm (Fig. 356, p. 413), finally reach special groups 

 of epithelial cells, which must no doubt be considered as sensory cells. 

 One principal branch of the oral (upper) pair, however, is said to run 

 to the musculature, uniting the joints of the arm. 



Besides these two pairs of nerves proceeding from the apical nerve 

 trunks, nerves are said to run out at the level of the joint, from 

 between the consecutive brachial joints, these having for their special 

 function the innervation of the brachial musculature. 



The apical pinnula nerves, which are given off in alternating order 

 by the brachial trunks to the pinnulas, arise from double roots. 



According to the present state of our knowledge on the subject, the 

 apical nervous system of the Crinoids is formed by the ccelomic endo- 

 thelium. Even in adult animals, those portions of this system which 

 envelop the chambered sinus, the stalk canal, and the cirrus canals 

 show a close connection with their places of formation. The apical 

 nervous system of the Adewids retains its endothelial position through- 

 out life. 



