462 COMPARATIVE ANATOMY CHAP. 



XII. The Sensory Organs. 



The sensory organs of the Echinodermata show, as compared with 

 the great complexity of the rest of their organisation, a very low 

 degree of differentiation. They are for the most part uridifferentiated 

 integumental (tactile) sensory organs. Specific sensory organs are 

 only rarely developed ; the oral feelers are considered to be olfactory 

 and gustatory organs ; the red spots at the tips of the arms of 

 Echinoids (on the terminal tentacles), and also the shining spots on 

 the integument of the Diadematidcc and related forms, are thought to 

 be eyes ; the sphseridia of the Eehinoidea, and Baur's vesicles 

 (otolith vesicles) in the Synaptidce and Elasipoda are considered to be 

 organs of hearing or of orientation. 



A. The Ambulaeral Appendages as Sensory Organs. 

 1. The Terminal Tentacles. 



We are undoubtedly justified in assuming that originally the radial 

 canals of the water vascular system in all Echinoderms ended distally 

 in a freely projecting tentacle or feeler, which, covered with a highly 

 developed sensory epithelium, functioned as a sensory organ. 



Such terminal tentacles are well developed in all Asteroids and 

 Ophiuroids, and are found in them at the tips of the arms, where, as 

 we have seen, the radial water vascular trunks end. They are sup- 

 ported by the terminal plates of the skeleton, and are surrounded by 

 small spines which, when the slightest stimulus is applied to the 

 terminal tentacle, bend towards one another protectively over it. 



It has long been known that the terminal tentacle of the Asteroids 

 carries a pigment spot, which has been regarded as an eye. 



In the Eehinoidea^ the terminal tentacle is already somewhat 

 reduced. The five tentacles are found on the five ocular plates of the 

 apical system, and the pore which perforates each ocular plate is the 

 aperture of the distal end of the water vascular trunk, which runs into 

 the tentacle. 



The five terminal tentacles are still further reduced in the Holo- 

 thurioidea, where they lie round the anus. 



Adult Crinoids have no terminal tentacles. The radial canals end 

 blindly before reaching the ends of the arms and pinnules. 



The terminal tentacles, in contradistinction to all other ambulacral 

 appendages (ambulacral feet, ambulacral tentacles, papilla), may be 

 regarded as primary appendages. In the youngest stages of the 

 animal, when the radial water vascular trunks have only just arisen 

 as outgrowths of the hydroccelomic vesicle, they lie close to the mouth. 

 They attain the position they occupy in the adult animal by the 



