VIII 



ECHINOBERM ATA— SENSORY ORGANS 



463 



growth and prolongation of the radial canals, during which process 

 bulgings arise alternately to right.and left, which, pushing the integu- 

 ment before them, form the ambulacral feet or tentacles. Those tube- 

 feet are therefore most recent in origin which are nearest to the 

 terminal tentacles and furthest from the ring canal. 



The terminal tentacle is the oldest ambulaeral appendage ; it 

 receives the distal end of the radial canal, and is the only unpaired 

 ambulaeral appendage of the radius. 



Special — 1. Asteroidea. — The tentacle whicli rests on the terminal plate is 

 covered by a very deep sensory epithelium, which consists of long, thin, sensory 



Fig. 873.— Water vascular system of a very young Asteroid. 1, Terminal tentacle ; 2, eye 

 spot at its base ; 3, ambulacral feet ; 4, radial canal of the water vascular system ; 5, circular canal ; 

 6, mouth. 



cells. It carries long cilia, and contains beneath the surface a thick layer of nerve 

 fibres, which represent the distal end of the radial nerve tract of the arm. At its 

 base, on the side turned to the mouth, it carries a vivid orange-red pigment spot. 



2. Ophiuroidea. — The tentacle is surrounded by the terminal plate as by a ring. 

 There is no eye. The subepithelial radial nerve enters the terminal tentacle and 

 ends in its epithelium. (These facts recall an analogous arrangement in the 

 Annelida, where the ventral cord still remains epithelial in the growing caudal end 

 after it has become subepithelial in other regions of the body). In the Euryalce, 

 which have much-branched arms, no terminal tentacles have been found. 



3. Eohinoidea. — The terminal tentacle in the adult is usually reduced to a low 

 papilla which rises above the pore in the radial plate. In Echinocyamus pusiUus 



