464 



COMPARATIVE ANATOMY 



CHAP. 



alone (Fig. 374) does this papilla project someAvhat further. The radial canal tra- 

 verses the pore and ends blindly under the epithelium of the papilla. The radial nerve 

 trunks also, and with them the epineural canal, pass through the pore. After reach- 

 ing the papilla, the fibres, Avhich 

 had hitherto been subepithelial, 

 enter the epithelium of the 

 papilla and the epineural sinus 

 ends. The pseudohsemal sinus, 

 on the contrary, accompanies 

 the radial canal and the radial 

 nerve trunk only to the point 

 where these latter enter the 

 pore. 



In a few rare cases the 

 radials are perforated by two 

 pores for the passage of two 

 terminal tentacles (Arbaciidce 

 and certain Palccechinoidea : 

 Melonitcs multipora, Palccc- 

 chinus elegans). 



4. Holothurioidea. In 

 Cucumaria cucumis and C. 

 Lacazii, the radial canals end 

 round the anus, just as in the 

 Echinoidca, but the last ex- 

 ternally visible trace of the 

 terminal tentacle has dis- 

 appeared. The radial canal perforates the body Avail, accompanied by the radial 

 nerve trunk and the epineural canal, and ends blindly close below the surface. The 

 radial nerve unites Avith those "nests" of cells Avhich represent the integumentary 

 epithelium and Avhich AA-ere described above, p. 415. The pseudohtemal canal ends at 

 the point Avhere the radial canal enters the body AA 7 all. 



In other Holothurioidea (e.g. Holothuria, impaticiis) eA'en the last (intrategu- 

 mentary) trace of the terminal tentacle is Avanting. The Synaptidce, haA r e no terminal 

 tentacle, since the radial canals are altogether Avanting in them. The distal ends of 

 the radial nerve trunks, however, perforate the integument, and in this AA T e may 

 perhaps see a last trace of the terminal tentacle. 



PIG. 374. Section through the terminal tentacle of 

 Echinocyamus pusillus (after Cue'not). 1, Body epithelium ; 

 2, test ; 3, epineural canal ; 4, radial nerve trunk ; 5, pseudo- 

 haemal canal ; 6, radial canal of the water vascular system ; 

 7, endothelium of the body cavity ; 8, genital circular sinus ; 

 9, terminal tentacle. 



2. The Ambulacral Feet and Ambulacral Tentacles. 



Experiments on the living animal shoAV that all the ambulacral appendages 

 are very sensitive to external stimuli, especially mechanical and tactile. If the 

 extended ambulacral foot of an Echinoid be irritated, it contracts, and the neighbour- 

 ing spines bend over it protectively. The same is the case with all the ambulacral 

 appendages of all Echinoderms. This is Avhat we should expect from the rich inner- 

 vation of these appendages, which, in addition to their other functions, must be 

 regarded as organs of touch. Tactile functions could indeed be safely attributed to 

 them by merely observing IIOAV the long, thin, suckerless feet at the end of the 

 Asteroid arm, or the feet on the anterior side of the Spatangoid body, stretch out 

 tentatively in various directions like the " horns " of a snail. 



It cannot yet be demonstrated that all or any of the ambulacral appendages of 

 the Echinodermata have any other special sensory functions. It has been con- 

 jectured that the oral tentacles are gustatory, but in the Synaptidce alone haA*e 



