viii ECHINODERMATA WATER VASCULAR SYSTEM 427 



tentacles and tentacle canals, the latter springing directly out of 

 the circular canal. 



The arrangement in all other Holothurioidea (Adinopoda) may be 

 described as follows. There are five radial canals, and never more. 

 The tentacle canals never spring directly from the circular canal, but 

 arise out of the radial canals. The tentacles are to be regarded as the 

 first (modified) tube-feet, and the tentacle canals as the first tube-feet 

 canals. 



The canals of the tube-feet and tentacles are usually connected 

 with ampullae. 



Actinopoda (Fig. 365). From the circular canal the radial canals run forward 

 (anteriorly) along the oesophagus towards the mouth, passing the axial surface of 

 the calcareous ring (i.e. between it and the oesophagus). They then pass, together 

 with the radial nerves on whose inner side they lie, through the incisions or apertures 

 belonging to them in the ring, and run backwards (aborally) in the body wall, 

 outside the circular musculature, and end blindly near the anus. 



In some rare cases, where the ventral surface is sharply distinguished from the 

 dorsal, and the dorsal ambulacral appendages, i.e. those of the bivium, have entirely 

 disappeared, the corresponding dorsal radial canals are said also to be wanting. In 

 a few isolated forms the central radial canal of the ventral side (i.e. of the trivium) 

 is also said to be wanting. 



The tentacle canals branch off from their radial canals just above the calcareous 

 ring. Their number corresponds with that of the tentacles to which they run. 

 These canals are often connected, at the anterior edge of the ring, with tentacle 

 ampullae (Fig. 365, 18). These latter are tubular outgrowths, which vary greatly in 

 size, stretching back over the outer surface of the calcareous ring, and for the most 

 part projecting freely into the body cavity. Where such ampullse occur, all the 

 canals without exception are provided with them. They are entirely wanting in the 

 families of the Elasipoda and Dendrochirota, but occur normally in the Synaptidce, 

 MolpadtidtBi and Aspidoclnrota. In Pelagothuria, branches run through the peculiar 

 swimming disc (cf. p. 286), radially, and reach even to the tips of its processes. 

 They are evidently to be regarded as modified tentacle ampullae. 



The canals of the tube-feet branch off alternately from the radial canals. As a 

 rule, a separate canal runs from the radial canal to each foot ; but in some cases 

 (Holothuria tubulosa} one canal, by branching, runs to several (4-6) tube-feet. In 

 the Molpadiidee, and the above-named Holothurian, it is said that there are tube-feet 

 canals which end blindly, and thus have no tube-feet answering to them. Except 

 in the footless Molpadiidee and the Psychropotidce, the tube-feet canals are connected 

 with egg-shaped, often somewhat long and occasionally branched ampullae. These 

 either lie as covered ampullae outside of the circular musculature of the body wall or, 

 as free ampullae, press in between the transverse musculature into the body cavity. 



At the point where the ampulla opens into the tube-foot canal, but in that part 

 of the latter which comes from the radial canal, there is a valve, similar to that 

 found in Asteroids, which will be described later. This valve is arranged in such a 

 way as to prevent the return of the fluid into the radial vessel, either from the foot 

 or from the ampullae. Valves are also found in the tentacle canals. 



The walls of the ampullae resemble those of the Polian vesicles in structure. 

 The radial canals and their branches are chiefly distinguished by the fact that the 

 longitudinal musculature is only developed in the outer part of the walls. 



Paractinopoda. The tentacle canals here spring directly out of the circular 

 canal, and nearly always agree in number with the tentacles. At the level of the 



