ECHINODERMS. 139 
The nervous system of Echinoderms was first described and 
figured by T1InDEMANN in Astertas aurantiaca (Astropecten). 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, KRouN dis- 
covered a few years ago a distribution similar to that of Astertw. 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 ?. 
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 im 
many species’. 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‘. 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 termima- 
tion. He conceives that the motions of the ambulacral tubes are 
1 In MecKEL’s Archiv f. die Physiol. 1. 1815, s. 161, &e. and in his often quoted 
prize essay. 
2 MvuettEr’s Archiv. 1841, pp. 1~-13, Tab. 1. 
3 Die Akalephen des rothen Meeres, s. 32—34, Tab. vill. fig. 11, 12. 
4 Comp. VALENTIN, op. cit. pp. 11, 100, Tab, 11, fig. 12, Tab, 1x. f. 188—rgo. 
