ECHINODERMS. 139 



The nervous system of Ecliinoderms was first described and 

 figured by TIEDEMANN in Asterias aurantiaca (Astropecten) l . There 

 is found around the mouth a nervous ring without ganglia, whence 

 is given off a fine thread for each ray, and running along it. On each 

 side of this thread is another, which descends into the cavity of the 

 body. In sea-urchins also and in star-fishes, in which TIEDEMANN 

 could only detect obscure traces of a nervous system, KROHN dis- 

 covered a few years ago a distribution similar to that of Asterice. In 

 Echinus the ring surrounds the mouth within the apparatus usually 

 named Aristotle's Lantern (see below in the systematic arrange- 

 ment) : in Holothuria in the calcareous ring to which the longitudinal 

 muscles are attached. Five principal nervous stems run with the 

 vessels that are in connexion with the ambulacral apparatus 2 . 



Little is known of special organs of sense in Echinoderms. In 

 star-fishes EHRENBERG discovered at the point of the rays on the 

 abdominal surface, a small red spot, surrounded by a ring of cal- 

 careous tubercles, which he considers to be an eye. In specimens 

 preserved in spirit the pigment disappears, and so the existence of 

 the spots cannot be recognized. Moreover they are wanting in 

 many species 3 . FORBES discovered five similar spots in sea-urchins, 

 on the upper surface, situated upon as many pentagonal plates that 

 alternate with five larger plates on which the oviducts open. Both 

 in the star-fish and sea-urchin each of the five principal nerves runs 

 as far as one of these spots and ends beneath it 4 . But in neither of 

 these animals has a lenticular transparent body been discovered. 

 The ambulacral tubes and the feelers around the mouth may, as 

 highly sensitive parts, be ranked amongst the organs of touch. 



To the motive apparatus of Echinoderms belong the little feet 

 or tentacles, already noticed, the ambulacral tubules by means of 

 which the animals creep. They have muscular fibres on their walls. 

 In Echinus VALENTIN found in them both transverse and longi- 

 tudinal bundles, and radiating fibres in the suckers at their termina- 

 tion. He conceives that the motions of the ambulacral tubes are 



1 In MECKEL'S Archiv f. die Physiol. i. 1815, s. 161, &c. and in his often quoted 

 prize essay. 



2 MUELLEE'S Archiv. 1841, pp. i 13, Tab. r. 



3 Die Akalepken des rothen Meeres, a. 32 34, Tab. vm. fig. n, 12. 



4 Comp. VALENTIN, op. cit. pp. n, 100, Tab. n. fig. 12, Tab. IX. /. 188 190. 



