MOLLUSCS. liJ7 



The most highly organized eyes in Annelids appear to 

 be those of the Alciopidas, which have been described 

 by Krohn,* de Quatrefages,t and especially by Greef } 

 and Graber.§ The Alciopidse are small 

 sea-worms ; they live principally in 3 

 the open sea, and, like many other S" 

 pelagic animals, are extremely trans- '^ 

 parent. It is, indeed, often difficult ^ 

 to see more of them than the two ^ 

 very large eyes, red or orann;e, and a 3 

 pair of dark violet dots (the seg- '— 

 mental organs) on each ring. ^ 



The principal parts of their eyes are ^ s^ % 

 — (l)theouter integument, the whole ^ ^ * 

 of which is so transparent that it needs ^ ^ ^ 

 scarcely any modification ; (2) the so- ''^'^}^'^ % 

 called " eye-skin," as to the true '^ 



nature of which there is still mucli J^ 



difference of opinion; (3) the lens; (4) ^ 



the "corpus ciliare ; " (5) the vitreous % 



humor ; and (6) the retina, which I 



again is composed of four layers — (a) / 



the rods; Qj) pigment layer; (e) \ 



granular layer ; {d) fibrous layer. pig^, 9i.-Aiciope (alter 



In Mollusca the eyes aie variously le Quatrer.g.s). 

 situated ; being, for instance, either placed on the pos- 

 terior tentacles ; or between the feelers, as in the fresh- 

 water species ; or on a short stalk at the side of the 



* "Zool. und Anat. Bemerk. iiber die Alciopeden," Wiegmann's 

 Arch., 184.'5. 



t " Etudes 8. 1. typ. Inf. de I'emb. des Annele's," j1«)i. Sci. Nat., 1850. 



t "Unt. iiber die AlciopiJen," Nova Acta Acad. Leop. Carpi, 

 voL xxxix. 11, 1876. 



§ Arch, fur Mlc. Anat, 1880. 



