VIII ECHINODERMATA — WATER VASCULAR SYSTEM 429 



2. Eehinoidea (Fig. 358, p. 419). — In the Spatangoida, where a 

 masticatory apparatus is wanting, the radial canals, on leaving the 

 circular canal which surrounds the mouth, are already in their 

 respective radii, and commence at once to send out branches right 

 and left to the tube-feet. In other Eehinoidea, however, the radial 

 canals have to descend from the circular canal which encircles the 

 oesophagus above the lantern to the peristome, and thus, after rising 

 out of the circular canal, have first to run under the intermediate 

 plates and over the intermaxillary musculature. They emerge at the 

 periphery of the lantern and then descend on its outer side, i.e. 

 outside the intermaxillary musculature to the peristome. Having 

 reached this latter, they first give off a branch which runs in the oral 

 region towards the mouth. They then pass through the auriculae, 

 in order to run up radially towards the apex, on the inner side of 

 the test, and in the middle lines of the ambulacra. They end 

 blindly in the pores of the radial plates of the apical system. 



While running up on the inner side of the test, the radial canals 

 give off alternating lateral branches, each of which enters an ampulla 

 (cf. Fig. 353, p. 410). The ampulla, which projects into the body 

 cavity, is itself connected, by means of one or two canals, with the 

 cavity of a tube-foot or tentacle, which latter projects freely on the 

 outer side of the test. The ambulacral plate at such a point is per- 

 forated by either a single or a double pore, according as the canal to the 

 tube-foot is single or double (cf. the section on the Skeletal System). 



In all Ecliinoidea, tlie tube-feet in young animals are all alike, and each is 

 connected with its ampulla by a single pore through tlie test. This may lie 

 considered to be the primitive arrangement. Tube-feet with single pores are found 

 in adults in a few Spatangoida : in the Pourtalesiidce, in the Ananchytidan genera 

 Urechinus, Cystechinws, Calyvnie, in the Spatangoid genus Palacotropus and the 

 Cassidulid genus Ncolampas. 



In all other Echinoids, the tube-feet or tentacles have double pores. In the 

 regular Echinoids {Cidaroida, Diadematoida), only double pores are found : but in 

 the Clypeastroida and the Spatangoida, only the pores of the petaloids are double : 

 those on the remaining ambulacral regions being single. 



The ampullfe are delicate structures which vary in shape. In cases in which 

 they, like the tentacles to which they belong, stand at some distance from one 

 another, they are pear-shaped or spherical ; but where they, like the tube-feet, stand 

 in compact rows in the ambulacral meridians, as in the regular Echinoids and in the 

 petaloids of the irregular forms, they are lengthened out horizontally and flattened 

 vertically (dorso-ventrally). The walls of the ampiiUie, from without inwards, 

 consist of: (1) a ciliated endothelium ; (2) a layer of connective tissue, containing 

 occasional enil^edded calcareous corpuscles ; (3) a circular muscle layer ; (4) an inner 

 ciliated epithelium. The lumen is traversed from wall to wall by fibres, which are 

 probably muscular. In several Echinoids, at the jioints where the lateral canals of 

 the radial canals open into the ampullae, valves have been observed. 



The branches of the radial canal which run in the oral integument supply the 

 tube-feet or tentacles occurring in this region. 



3. Asteroidea. — The radial canals, in this class, run along the 



