FEATHER STARS. 



245 



is also represented in starfishes. Apart from the superficial epithelium 

 there are no sensory structures. 



The ciliated food canal descends from the mouth into the cup, and 

 curves up again to the anus, which is usually ex-centric in position. The 

 last part of the gut is expanded to form an anal tube, which during life 

 is in const.ant movement, and has apparently a respiratory function. 

 From the cup, where the body cavity is in great part filled with con- 

 nective tissue and organs, two ccelomic canals extend into each of the 

 arms. They communicate at the apices of the arms and pinnules, and 

 currents pass up one and down the other. 



The blood vascular system consists of a circumoral ring, which is con- 

 nected with a radial vessel under each ambulacral nerve, and with a 

 circumcesophageal plexus. There is also a " plexiform organ," " lying 

 interradially in the disc anteriorly to the mouth " (g in Fig. 79). It en- 

 closes the barren central part of the reproductive system (the axial genital 



Fig. 79. — Diagrammatic vertical section through disc 

 and base of one of the arms of Antedon Rosaceus. (After 

 MiLNES Marshall.) 



The section is interradial on the left, radial on the right. ?., 

 ciliated openings in body wall ; ^., subepithelial ambulacral nerve; 

 /., water vascular cana! ; /cr., tentacle; r., mouth ; ^., intestine ; ^., 

 central plexus, with "chambered organ" at its base; Ri.-K;^., 

 radial plates ; Br., brachial plates ; w., muscle ; a., axial nerve cord ; 

 d., central capsule ; CD., centro-dorsal plate ; /., cirri ; e., branches 

 from central capsule to cirri. 



stolon), and has connections with the above mentioned plexus, with the 

 vessels to the organs, and with a strange "chambered organ " which lies 

 within the central aboral nervous system. 



The water vascular system consists as usual of a circumoral ring and 

 radial vessels. These lie under the corresponding blood vascular system. 

 But the system is divergent in several ways ; {a) water passes into it by 

 several ciliated and branched water tubes which hang from the ring, and 

 from the origins of the radial vessels, into the body cavity ; (i) water 



