284 



PHYLUM ECHINODERMATA. 



of adjacent radii by a commissure which forms a ring round 

 the cup (Fig. 196). 



In Encrinus and Pentacrinus the apical nerve cords are double. In 

 many Palaeozoic Crinoids the canals for these axial cords are not separ- 

 ated from the ventral grooves on the brachials. In others they are present 

 in the brachials, but exist only as grooves on the radials and basals. 



The apical nerves give off branches which ramify in the ossicles 

 (Fig. 197), and supply the muscles of the ossicles and the inte- 

 gument ; some of them are connected 

 with the branches of the deep oral 

 system. A prolongation of the ner- 

 vous sheath, which surrounds the 

 chambered organ and forms the 

 centre of this system, accompanies 

 the prolongation of the chambered 

 organ into the stalk and cirri, or if 

 the stalk is absent into the cirri of 

 the centro-dorsal plate. 



FIG. 196. Antedon bifida. Diagram 

 showing the arrangement of the 

 apical nervous system in the calyx, 

 aaxial cords, the black disc in the 

 centre of the figure represents the 

 central sheath which surrounds 

 the chambered organ ; CD centro- 

 dorsal plate ; B\ first secundi- 

 brach ; RI radial ; #2, -#3 primi- 

 brachs (from Perrier after Lud- 

 wig). 



As in Asterids the apical nervous sys- 

 tem appears to originate in connexion 

 with the coelomic epithelium. It was dis- 

 covered by Dr. W. B. Carpenter * in 

 Comatula as a result of his experiments 

 on the animal. His observations were for 

 some time discredited, but were eventually 

 confirmed by Marshall ,f who showed that 



the nervous aggregation round the cham- 

 bered organ governs the movements of the arms and that the nerve cords 

 proceeding from it contain both sensory and motor fibres. 



The deep oral nervous system is placed below the epithelium ; 

 it consists of a ring round the mouth and two radial nerves in 

 each arm (Fig. 197, 4). The ring also gives off nerves which 

 ramify in the connective tissue strands of the body-cavity. 

 Branches of the brachial nerves of this system anastomose with 

 branches of the apical cords. 



Sense organs. There are no terminal tentacles nor special 

 organs of sense. The tube-feet must be regarded as specially 

 sensitive and possibly the whole arm as well. The tube-feet are 

 supplied by both the superficial and the deep oral nervous system. 



* Proc. Roy. ,S'oc. 24, 1876, p. 211, and vol. 37, 1884, p. 67. 

 t Q.J.M.S., 24, 1884, p. 507. 



