92 THE VOYAGE OF H.M.S. CHALLENGER. 
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 
Fic. 3.—Diagram showing the course of the Digestive Tube in an Actinometra with Interradial 
Mouth, as seen from the ventral side. 
Letters as in fig. 2. The + 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 Promacho- 
erinus, 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 ampulle 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 by 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 off 
their branches to the tentacular groups in the usual regular way (Pl. Ve. fig. 7. 
Pl LVEL. figs, 1) 35.42 PII ies, 1,5 Pi, Tk figs. 1,2, 6—tb; Pl. LXI. figs. 4, 6). 
1 Crinoideen, Loc. cit., p. 337. 2 See also fig. 5 on p. 121, m’. 
