142 



THE EYE IN EVOLUTION 



Buccinum 



Helix 



the vesicle are partly light-sensitive, partly secretory, the former being 

 frequently associated with pigment and connected by nerve fibres with 

 the oi^tic or cerebral ganglion ; the distal (superficial) elements are rela- 

 tively undifferentiated, and a refractile mass of secreted material, 

 homologous M'itli the vitreous of higher types, fills the cavity. Such a 

 simple ocellus, lying in the subepithelial tissues over which the 

 epithelium passes without interruption, is seen most particularly in 

 Gastropods such as Murex which furnished the Tyrian purple,^ the 

 common whelk, Buccinum, or the edible snail, Helix pomatia (Fig. 

 110). 



Its most elaborate form is seen in tlie spider- or scorpion-shell, Pterocera 

 lamhls, a gastropod found on tropical reefs, wherein the vesicle, filled with a 

 vitreous-like material, has a clear tlistal wall (a cornea), while the proximal part 



a 



i'iu. 111. — TiiK Ketina of Ptehoceua lambis. 



The retina contains four layers : (a) a layer of rods ; (b) a layer of pigment 

 cells containing some rod nuclei ; (c) a cellular layer in which are distin- 

 guishable most of the rod nuclei, bipolar cells, a few horizontal cells, ganglion 

 cells and supporting cells with a reticulum resembling Midler's fibres in tlie 

 vertebrate retina ; (d) a layer of optic ner\e fibres (J. H. Prince). 



of the vesicle is occupied by a retina consisting, according to Prince (1955), of 

 4 layers — (a) most distally, a layer of rod -like visvial cells, (6) a layer of pigment 

 cells, (c) a cellular layer containing the nuclei of the rods, synaptic "bipolar", 

 " horizontal " and ganglion cells, and {d) a layer of ojjtic nerve fibres, the axons 

 of the ganglion cells which leave the eye in nvimerous optic nerve bundles 

 (Figs. Ill and 189). With a receptor population ajaproaching 10,000 per scj. mm., 

 the sensitivity of the eye is j^robably considerable although, in the absence of an 

 efficient optical system, image -format ion must be verj^ deficient. 



In a further stage of complexity a lens is added to the vesicular 

 eye so as to form a camera-like eye resembling that of vertebrates ; 

 an accommodative mechanism and an extra-ocular musculature are 

 provided. This is typically seen in two very different phyla : among 

 the Polychsetes in the family of Alciopidae, and among the Cephalopods 

 whic> have the most elaborate eyes in the invertebrate kingdom. 



^ Set? Singer, The Earliest Chemical Industry, London, pp. 12-14 (1948). 



