THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 265 



structure. Running along the ventral surface of each arm is a longi- 

 tudinal furrow, the ventral or ambulacra! groove (fig. 2, i), bordered on 

 either side by a fold of perisorne, 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 ambulacra! epithelium 

 which is columnar and ciliated and 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 longitudi- 

 nally, and so appearing as fine dots in transverse sections of the arm : 

 interspersed among the fibrils are very small nucleated cells. The 

 subepithelial band in Antedon rosaceus is continuous with the ambu- 

 lacral 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. lu 

 other species the subepithelial 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 ambulacra! 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 the subepithelial bands 

 of the five radial grooves unite to form a pentagonal ring. 



The tentacles, as described above, are hollow ; their cavities com- 

 municate with a longitudinal canal (fig. 2, I) which runs along the arm 

 just below the subepithelial band. These radial ambulacral canals are 

 continued into the disc and open into a circular canal round the mouth, 

 (fig. 1) from which short branching canals are given off ending in open 

 mouths communicating with the body cavity. 



Besides the radial ambulacral canal, each arm contains also three 

 diverticula of the body cavity or coelom. Of these the' most ventrally 

 situated (fig. 2, m) is called the subtentacular canal and is commonly 

 divided, as in the figure, by a median vertical partition ; the most 

 dorsally placed canal (fig. 2, n) is called the cceliac and communicates 

 at the end of the arm with the subtentacidar. The third or genital 

 canal is placed between the other two and lodges the cord-like genital 

 gland ; it is very small in the arm, but much larger in the pinnules. 



