26 



sections of the arms in which a nerve appears in the 

 position indicated by Cuenot it may be seen to give o&. 

 twigs to the ambulacra! tentacles and to others which 

 run towards the dorsal face of the arm (PL III., fig. 29, 

 It. nv. c.) where they are said to unite with similar twigs 

 from the brachial cords of the apical system. Moreover, 

 it has not been shown how the single pair of nerve cords 

 which are said to run alongside the ambulacral grooves of 

 the tegmen calycis divide at the point of origin of each 

 pair of arms, so as to furnish a lateral nerve cord to their 

 inner faces. Nerves from the circum-oesophageal ring 

 are traceable to the oral tentacles (PL Y., fig. 52), and two 

 cords traverse the anal interradius to innervate the anal 

 funnel. Similar nerves occur in the other interradii, and 

 their branches appear to anastomose, thus forming a sub- 

 tegumentary plexus. From the latter, nerve twigs go to 

 innervate the bands and trabeculse of connective tissue in 

 the body cavity. It is highly probable that this sub- 

 epithelial system is in continuity with the su^Derficial 

 system as well as with the apical system. 



The Apical system consists of a cup-shaped structure, 

 the central capsule, from which stout nein^e cords proceed 

 and eventually unite in pairs to form the apical or dorsal 

 nerve cords of the arms (PL Y., figs. 48, 49, and 52; 

 PL YI., figs. 54, 55, 56, 57, and 59, cen. caj).). The 

 central capsule is lodged in the concavity of the centro- 

 dorsal plate, to the walls of which it is closely applied, 

 and its oral or ventral face is covered by the rosette plate 

 (PL YI., fig. 59). Its thick walls form a close investment 

 around the chambered organ ; and, as has been already 

 stated above, the cirrus vessels which radiate from the 

 chambers are also invested by tubular extensions of the 

 capsule which form the so-called cirrus cords (figs. 52 

 and 59). 



