19 



The peripheral cavity in like manner communicates 

 with the dorsal or coeliac canals of the same (figs. 29, 

 41, and 52, coeca.). These canals are, therefore, the 

 ccplomic cavities of tlie arms, and are lined by coelomic 

 endothelium (fig. 41, cce. en.). They traverse the arms 

 and pinnules to their extremities, and communicate 

 with each other at various points by minute openings in 

 the horizontal and vertical septa which bound them. 

 When viewed in a transverse section of an arm (fig. 29), 

 the dorsal or coeliac canal {cce. ca.) appears as a sub- 

 triangular space, bounded dorso-laterally by the flexor 

 muscles and ventrally by a horizontal septum. The latter 

 forms also the dorsal boundary of the subtentacular 

 canals, two quadrant-shaped spaces [s. te. ca.) separated by 

 a vertical septum, which, however, disappears before the 

 canals enter the disc. Immediately below the point where 

 the vertical septum merges into the horizontal there is a 

 small rounded or lenticular space, the genital lacuna 

 (fig. 41, gen. Ic), which encloses the genital cord or rachis. 

 Ciliated Pits. — In the median sagittal line of the 

 dorsal wall of the coeliac canal of the pinnules, and 

 immediately above the nerve cord, there are groups of two 

 to six bowl-shaped depressions with slightly raised 

 margins, and lined by modified cells of the coelomic 

 endothelium (PI. III., fig. 37, cil. jjt.). The cells which 

 line the bottom of the depression are flattened and non- 

 ciliated, while those which line the sides are columnar 

 and ciliated. The action of the cilia doubtless serves to 

 promote currents in the coelomic fluid, and on this 

 account the ciliated pits of Antedon have been compared 

 with the ciliated funnels of the Synaptidse and the ciliated 

 bands in the brachial cavities of the Ophiuroidea. Should 

 this be their only function their comparative rarity in the 

 coeliac canals of the arms is remarkable. Their linear 



