192 



THE EYE IN EVOLUTION 



Branchiomma 



in certain pelagic polychsetes such as Alciopa and Eupolyodonfes, the 

 intimate structure of which has already been described. ^ These worms 

 have two eyes, sometimes facing forwards {Eupolyodontes), sometimes 

 diverging widely (Alciopa) (Fig. 174). Each organ is provided with 



an elaborate retina, a lens, an accom- 

 modative mechanism and extra-ocular 

 muscles suggesting the potentiality for 

 binocular vision, an equipment which 

 seems capable of considerable visual 

 powers approximating those of the 

 Cephalopods.2 Little, however, is known 

 of the habits of these worms. 



In the sedentary tubicolous poly- 

 chsetes (Potamilla, Branchiomma, Dasy- 

 chone, etc.) the ocelli are frequently 

 grouped in masses on the branchial fila- 

 ments to form a composite simple eye 

 of great complexity (Brunotte, 1888 ; 

 Andrews, 1891 ; Hesse, 1896) (Figs. 175 and 176) ; Vermilia infundi- 

 bulum has at least 220 ocelli on the external aspect of each branchium, 

 a total of some 11,000 eyes (Parker and Haswell, 1940). These 

 creatures live within their tubes from out of which extend the branchial 

 plumes bearing the filaments on each of which there is one or more 

 such eyes (Figs. 128, 129). The curious thing, however, is that in 



Figs. 175 and 176. — The Complex Eyes of Tubicolous Polych^tes. 



e 



Fig. 174. — The Anterior End of 

 THE Polych^te Wokm, Alciopa. 



Showing the two large eyes (after 

 Greeff). 



Fig. 175. 



Fig. 176. 



The secondary filaments are seen issuing horizontally from the central 

 axis of the branchial filament. Fig. 175, Branchiomma, showing the single 

 complex eye, e, near the termination of the central axis. Fig. 176, Dasychone, 

 showing the row of complex eyes (2 of which are marked e) running up and 

 ';osvn the central axis (after Benham, Camh. Nat. Hist.). 



143. 



Fig. 112. 



