THE STIMPLE EYE 



143 



The remarkable eyes of the Alciopidse, a family of pelagic polychsetes 

 {Alciopa, Vanadis, etc.), have received considerable study ^ (Fig. 112). In these 

 worms the proximal part of the vesicle is occupied by a retina with direct 

 receptors ; the main body of the vesicle contains a vitreous-like mass of two 

 consistencies, separating the retina from the anteriorly situated lens. The 

 posterior portion of the vitreous is jelly-like and is secreted by the intercalary 

 cells of the retina ; the distal portion is derived from a glandular cell situated 

 ventrally. There is an effective accommodative mechanism - and the eyes are 

 moved by 3 extrinsic muscles. Nothing is known about the function of these 

 elaborate eves. 



Fig. 112. — ^The Eye of the Polych.ete Worm, Va.\adi^. 



BV, blood vessels ; CT, connective tissue ; DV, distal vitreous ; G, 

 ganglion cells ; GC, glandular cell secreting the distal vitreous ; L, lens ; 

 ON, optic nerve ; NF, optic nerve fibres ; PR, proxiinal retina ; PV, proximal 

 vitreous ; R, main retina showing the rods separated from the visual cell-bodies 

 by a dense line of pigment (after Hesse). 



The eyes of the dibranchiate cephalopods (cuttlefish, squids, octojxis, etc.) 

 have received a considerable amount of study (Figs. 113, 114).^ The two eyes 

 are set on pedicles on either side of the head, and are partly enclosed in a dense 

 supporting envelope reinforced with cartilage. The vesicle is filled with a 

 vitreous secretion ; the cells lining its proximal portion form the retina ; the 

 distal portion fuses with an invagination of the surface epithelium to form a 

 composite spheroidal lens, the inner half of which is thus made up of vesicular 

 epitheliuiB, the superficial half of surface epithelium. On either side of the lens 

 the fusion of these two layers forms a doiible epithelial layer — a " ciliary body " 

 — and then the surface epithelium turns upon itself to form an " iris " before 



1 Greef, 187.5-77 ; Demoll, 1909 ; v. Hess, 1918 ; Pflugfelder, 1932. 



« p. 591. 



3 See Scarpa (1789), Cuvier (1817), Soemmerring (1818), Krohn (1835-42), Hensen 

 (1865), Schultze (1869), Patten (1886), Carriere (1889), Grenacher (1895), Hesse (1900-2), 

 Merton (1905), Butschli (1921), Alexandrowicz (1927), Heidermanns (1928), and others. 



