274 



NATURAL HISTORY. 



Fi 



gradually disappear from the exterior of the calyx ; though in one rare genus and in most Stalked 

 Crinoids they are visible beneath the radials, alternating with them in position, and cutting them 

 off more or less completely from the top stem-joint. 



Jointed on to the first radials, and attached to them by muscles, are the five second radials 



(Fig. 22, R 2 ), each in- its turn bearing a third 

 or axillary radial (R S ). The outer face of this 

 is roof-shaped, and bears the lowest joints (B) 

 of two arms, which may or may not fork 

 again. 



Immediately beneath the ciliated am- 

 bulacral or food-groove of each arm and 

 pinnule lies a nervous band (Fig. 21, n), and 

 deeper still, a blood-vessel (b), just as in the 

 groove on the under-side of a Starfish arm 

 (Fig. 9, n, b). Beneath the blood-vessel is 

 the water-vessel (Fig. 21, w), which gives off 

 side branches to the tentacles (T). These are 

 delicate tubular organs, which are situated at 

 the sides of the food-groove, and correspond 

 to the tube-feet of the Starfish (Fig. 9, t). 

 Not being required for locomotion, they are 

 essentially breathing organs. 



The water-vessels of the arms unite into 

 five trunks which communicate, like those of 

 the Starfish arms, with a ring-shaped vessel (Fig. 

 20, wr\ situated in the lip around the mouth. 

 There is, however, no such direct communica- 

 tion between this vessel and the external water as is effected by the stone-canal or water-tube of 

 the Echinozoa. But water is able to enter the body-cavity by innumerable small tubular openings 

 in its walls, the 

 water-pores (Fig. 

 20, wp). These 

 are lined by cilia, 

 all working in- 

 wards ; and, on 

 the other hand, 

 the water-vas- 

 cular ring is in 

 free communica- 

 tion with the wi 

 body-cavity by 

 means of delicate 

 ciliated tubuli 

 the water-tubes 

 (Fig. 20, wt), 

 the open ends of 

 ivhich hang down 



jnto it, and thus Fig. 20. LONGITUDINAL SECTION THROUGH THE HODY OF AN IRREGULAR FEATHER-STAR (Comatula 

 establish an in- 



i . .a, axial cords of the rays : oe, gullet ; a<t, ambulacra! groov 



direct COmmUni- joints; C, body-cavity, chiefly occupied by eonnectiv ' 



19. ROSY FEATHER-STAR (Comatula rosaceci), VIEAVED FROM 



AliOVE. NATURAL SIZE. 



The food grooves of the arms are seen converging towards the nearly central 

 mouth, immediately below which is the projecting anal tube. 



parvicirra) . MAGNIFIED EIGHT TIMES. 



cation between 

 the body-cavity 



L ' " 1): 'P', lal) ' nl 



L ' " 1): ' 



am, branch of axial cord to muscle; at. anal tube; Br, arm- 

 ue ; crl, centro-rtoreal ; eh, chambered organ; ci, cirrus ; civ, 

 ,. . 'enital vessel : ib, blood-vascular network over digestive tube ; 

 muscle: A r , flbrillar envelope of the chambered organ ; nr, oral nervous ring; ob, 



. . . --, , .. _ _. 



cirrus vessel ; cp, central plexus; (7, digestive tube : <i 

 Ip, labial plexus ; M, mou 

 od-vascular ring ; SI, first radial ; wp, water-pore ; wr, water-vascular ring ; wt, water-tube. 



and the exterior. Between the dorsal skeleton of the arms and pinnules (Fig 21, pj) and the water- 



