MOLLUSCS. 137 



The most highly organized eyes in Annelids appear to 

 be those of the Alciopidae, which have been described 

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

 and Graber.§ The Alciopidse are small 

 sea-worms ; they* live principally in 5 

 the open sea, and, like many other J^ 

 pelagic animals, are extremely trans- ^ 

 parent. It is, indeed, often difficult ^ 

 to see more of them than the two ;3^ 

 very large eyes, red or orange, and a ^ 

 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 

 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 much 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) f 



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



granular layer ; (d) fibrous layer. ng. gi.-Aiciope (alter 



In Mollusca the eyes are variously de Quatrefages). 

 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. imd Anat. Bemerk. iiber die Alciopeden," Wiegmann's 

 Arch., 1845. 



t " Etudes s. 1. typ. Inf. de I'emb. des AnneUs;' Ann. Sci. Nat, 1850. 



J/'Unt. iiber die Alciopideu," Nova Ada Acad. Leop. Carol, 

 vol. xxxix. 11, 1876. 



§ Arch, fur Mic. Anat., 1880. 



