92 THE VOYAGE OF H.M.S. CHALLENC4ER. 



tube (PL LXI. fig. 1). This is often some little distance in front of the point where the 

 lowest part of the long gullet turns off westwards to enter the great outside coil. The 

 walls of this long digestive tube are tolerably simple and but slightly plicated. For an 



E 



D 



Fig. 3.— Diagram showing the course of the Digestive Tube in an Actinomclro with Iiiterrailial 



Mouth, as seen from the ventral side. 



Letters as in fig. 2. Tlie + on the hinder portion of the outer coil indicates the limit of that part of the gut which 



corresponds to the entire digestive tube (excepting the rectum) of an endocyclic Crinoid. 



extensive secreting surface is amply provided, without the necessity of this further 

 complication, which is so largely developed in the simple spire of the gut in Promachu- 

 crinus, Antedon eschrichti, and Antedon antarctica. 



C. The Water- Vascular System. 



The water- vascular system of a Crinoid consists, like that of the Stellerids, of an oral 

 ring and radial vessels, the former being connected indirectly with the exterior through 

 the intervention of the water-tubes, water-pores, and the body-cavity. Neither the oral 

 ring nor the radial vessels have any ampuUge connected with them ; though, as suggested 

 by Ludwig,^ these are perhaps represented by the small lateral pouches of the radial vessels 

 which are opposite to the tentacular branches, and are crossed hj muscle-threads'" 

 (PL LX. fig. 6). 



The presence of these tentacular branches is invariably correlated with that of the 

 food-groove. Where this is well marked, and lined by ciliated epithelium with the 

 subjacent ambulacral nerve and blood-vessel, the water- vessels beneath the latter give ofi" 

 their branches to the tentacular groups in the usual regular way (PL Vc. fig. 7. 

 PL LVIL figs. 1, 3, 4 ; PL LIX. fig.s. 1,5; PL LX. figs. 1, 2, 6—tb ; PL LXI. figs. 4, 6). 



' Crinoideen, Ice. cif., p. .337. - Sec also fig. 5 on p. 121, m'. 



