NERVOUS SYSTEM OF ANTEDON KOSACEUS. 509 



The several joints of the arms are moveable on one another. 

 Movement towards the oral or ventral surface^ which will be 

 called flexion, is effected by muscles (figs. 1 and 2, u) running 

 between the successive segments; extension or movement 

 towards the dorsal surface is on the other hand almost entirely 

 due to the action of elastic ligaments placed nearer the dorsal 

 surfaces of the segments. 



The dorsal and lateral surfaces of the arms are covered by an 

 extremely thin layer of integument, but along the ventral 

 surface the soft parts are much thicker and exhibit a consider- 

 able complexity of structure. Running along the ventral sur- 

 face of each arm is a longitudinal furrow, the ventral or 

 ambulacral groove (fig. 2, i), bordered on either side by 

 a fold of perisome, the edge of which is notched into a series of 

 concentric leaflets at the base of each of which is a group of 

 three hollow tentacles (k). 



The ambulacral groove is lined by a special ambulacral 

 epithelium which is columnar and ciliated arid much thicker 

 than the non- ciliated epithelium covering the rest of the body. 

 Beneath the columnar cells is a fibrillar layer (fig. 2, h) spoken 

 of as the subepithelial band. This consists of very slender 

 fibrils arranged for the most part longitudinally, and so appearing 

 as fine dots in transverse sections of the arm; interspersed 

 among the fibrils are very small nucleated cells. The sub- 

 epithelial band in Antedon rosaceus is continuous with the 

 ambulacral epithelium, of which it may be described as forming 

 the deepest layer; it is traversed vertically by strands which 

 are continuous on the one hand with certain cells of the 

 columnar epithelium, and on the other with a connective-tissue 

 stratum underlying the band. In other species the subepi- 

 thelial band appears from the descriptions of Ludwig and others 

 to be separated from the ambulacral epithelium by a very 

 thin connective-tissue lamella. 



At the bases of the arms the ambulacral grooves are continued 

 on to the disc ; those of each pair of arms unite together and 

 so give rise to five radial grooves which run over the surface of 

 the disc to the mouth, where they meet. Round the mouth 



