464 



COMPARATIVE AXATOMY 



CHAP. 



alone (Fig. 374) does this papilla project somewhat 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, which 

 had hitherto been subepithelial, 

 enter the ejiithelium of the 

 papilla and the epineural sinus 

 ends. The pseudohjemal 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 (Arhaeiuhc 

 and certain I'a/cccchiiioidca : 

 Melonites nmiUipora, Palcec- 

 chiiius ehyans). 



i. Holothurioidea. — In 

 Citcumaria ci'cinnis and C. 

 Laciaii, the radial canals end 

 round the aims, just as in the 

 Ecliiiiid'tcfi, hut the last ex- 

 ternally visible trace of the 

 terminal tentacle has dis- 

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

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

 radial nerve unites with those "nests" of cells which represent the integumentary 

 epithelium and which were described above, p. 415. The pseudohremal canal ends at 

 the point where the radial canal enters the body wall. 



In other Holothinioidea (e.g. Holotliuria iinpaticns) even the last (intrategu- 

 mentary) trace of the terminal tentacle is wanting. The Synaptida: have no terminal 

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

 the radial nerve trunks, however, perforate the integument, and in this we may 

 perhaps see a last trace of the terminal tentacle. 



Fk;. 374.— Section through the terminal tentacle of 

 Echinocyamus pusillus (aftei- Cu^not). 1, Body epithelium ; 

 ■J, test ; 3, epineural canal ; 4, radial nerve trunk ; 5, iiseudo- 

 liienial canal ; G, radial canal of the water vascular sy.steni ; 

 7, endotlieliuHi of the body cavity ; S, genital circular sinus ; 

 9, terminal tentacle. 



2. The Ambulacral Feet and Ambulacral Tentacles. 



Experiments on the living animal show 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 Ecliinoderms. This is what 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 how 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 Synaptidm alone have 



