MOLLUSCS. 137 



The most highly organized eyes in Annelids appear to 

 be those of the AlciopidsB, which have been described 

 by Krolm,* de Quatrefages,t and especially by Greef | 

 and Graber.§ The Alciopidse are small 

 sea-worms ; they live principally in J 

 the open sea, and, like many other 

 pelagic animals, are extremely trans- ^ 

 parent. It is, indeed, often difiQcult ^ 

 to see more of them than the two ^ 

 very large eyes, red or orange, and a S^ 

 pair of dark violet dots (the seg- ^ 

 mental organs) on each ring. ^ 



The principal parts of their eyes are ^ ^ % 

 — (1) the outer integument, the whole ^J ^ J 

 of which is so transparent that it needs ^ ^ '"^ 

 scarcely any modification ; (2) the so- '^^H0 

 called "eye-skin," as to the true 

 nature of which there is still much ^ 



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



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



humor; and (6) the retina, which | 



again is composed of four layers — (a) f 



the rods; (h) pigment layer; (c) \ 



granular layer ; (d) fibrous layer. rig. 9i.-Aiciope (alter 



In Mollusca the eyes are variously '' Quatrcfages). 

 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," Wiegmami's 

 Arch., 1845. 



t " Etudes s. 1. typ. luf. de I'emb. des Annele's," Ann. Set. Nat, 1850. 



X "Unt. iiber die Alciopiden," Nova Acta Acad. Leop: Carol, 

 vol. xxxix. 11, 1876. 



§ Arch, fur Mia. Anat, 1880. 



