ECHINODERMATA. 



195 



mouth (g) immediately exterior to the circular chylaqueous vessel : 

 from this nervous ring three delicate filaments are sent off opposite 



Section of Asteracantiiion rubens, showing the tubular feet. 



the hase of each ray : the lateral filaments enter the discoid body : 

 the middle one is continued along the ambulacral groove, and swells, 

 according to Ehrenberg, into a small terminal ganglion, immediately 

 behind that bright-coloured speck at the extremity of the ray which 

 the same acute observer regards as a rudimental organ of vision (b). 



This pigmental organ manifests itself at the earliest appearance of 

 the arms, when they form five mai'ginal lobes of the discoid body of 

 the young star-fish, and give it the pentagonal form which is retained 

 throughout life in the A. discoidea.* 



It is objected by the laborious Siebold that no dioptric apparatus 

 (keine deutliche lichtbrechende Korper) is connected with the pig- 

 ment-speck ; nor has any better reason been given for the visual 

 function of that speck than its relation to a subjacent mass of nervous 

 matter, the analogy of the accumulation of pigmental matter in the 

 choroid of true eyes, the position of the pigment-specks at the fore- 

 part of the body in Rotifers, Planariae, Borlasiae, Leeches, 8sc., and the 

 obvious proof that such simple pigment-speck must absorb those rays 



* CLXIV. p. 172. 

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