430 COMPARATIVE AXATOMY chap. 



bases of the ambulacral furrows of the arms, outside the ambulacral 

 phites. At the tips of the arms they end blindly in the terminal 

 ocular tentacles. In their courses, consecutive widenings and 

 narrowings are not infrequently found, these correspond \rith the 

 segmentation of the arm, but are never very marked. Each radial 

 canal gives off — at regular intervals, which correspond with the 

 skeletal segments, and at opposite points to right and left — eanals to 

 the tube-feet. At the point where such a canal opens into the tube- 

 foot, a second canal, the ampulla eanal, branches oft' from it. This 

 canal rises up between two consecutive ambulacral plates to widen out 

 above these latter into an ampulla which projects freely into the body 

 cavity (Fig. 354, p. 411). 



This ampulla is single in all J'oung Asteroids and many adults (Linckiidn, 

 Ech blaster idee, Astcriida, Luidia). In other Asteroids (Astropedinidcc excluding 

 Luidia, Asteriaidce, Pi iitacerotido_\ c;/. Ciilcila) two separate ampulhi; occui' to 

 each tube-foot in the adult. 



Valves are found at the points where the canals of the tube-feet 

 open into the radial canal. A muscular membrane, resembling a 

 truncated cone with the base attached horizontally round the wall of 

 the canal, projects into the lumen directed towards the foot. This 

 valve prevents the fluid pressed out of the ampulla from returning 

 into the radial canal, either because the membrane is able by muscular 

 action to close the aperture, or because the pocket surrounding this 

 projecting membrane is swelled up by pressure of water from the 

 foot or ampulla, and so closes the valve. 



4. Ophiuroidea. — The first point to be noted with regard to 

 the Ophiuroidea is that they have no tube-feet ampuUaj. 



The radial trunks of the water vaseulap system run in the arms 

 between the ventral shields and the vertebral ossicles. At the tip 

 of the arm each trunk ends in a small terminal tentacle. Eegularly 

 consecutive and distinct widenings are found in their courses 

 corresponding with the regular segmentation of the arms. Between 

 every two of these consecutive widenings, the radial canal is provided 

 with a single layer of band-like circular muscle fibres. A narrow 

 tube-foot canal runs off to right and left from each widening, running 

 either straight into its tentacle or first forming a V-shaped loop, which 

 ascends apically into the calcareous mass of the vertebral ossicle. At 

 the point where the tentacle canal enters the tentacle, the lumen of 

 the former becomes much widened, and a valve occurs (similar to that 

 described in the Asicroids), which prevents a flowing back of the 

 water vascular fluid out of the tube-foot into the radial canal. 



The first two pairs of canals to the tube-feet or tentacles (the so- 

 called oral tentacles) come direct from the circular canal. 



5. Crinoidea. — Tentacle ampullae are wantingr. The radial 

 canals lie close under the food grooves of the disc, of the arms, and of 

 the pinnulse, whose courses they exactly follow, so that they branch just 



