THE SIMPLE EYE 



139 



depression or dimple in the epithelium, such as is seen typically among 

 Molluscs ; some 30 such cup-shaped depressions, for example, each 

 ^ mm. in diameter, skirt the border of the mantle of the bivalve, Lima, 

 while similar structures are seen at the base of the tentacles of the 

 common limpet. Patella (Fig. 97). The simple eyes of the larva of the 

 house-fly, Musca, are of a similar type (Bolwig, 1946) (Fig. 99).^ In such 

 cases the sensory epithelium may be composed of light-sensitive pig- 

 mented cells interspersed with unpigmented secretory cells which secrete 

 a protective material covering the epithelium. The second stage is 

 marked by an overlapping of the surface epithelium so that the shallow 

 pit becomes converted into a cavity with a tiny opening. Such a cup 

 may be oval and deep and filled with secretion, as in the ear-shell, 

 Haliotis (Fig. 98), but the tendency is seen in its most marked form in 

 the rare pearly mollusc. Xaufilus. which lives in a beautiful spiral shell 

 in the seas of the Far East (Fig. 100). In this cephalopod, situated 

 just behind the tentacles, a pin-hole opening 2 mm. in diameter 

 leads into a large ocular cavity lined by light-sensitive cells bathed by 

 sea-water, the eye thus constituting a veritable dark chamber (Merton, 

 1905). In a third and final development the cavity is closed by the 

 growth of the cuticle associated with hypodermal cells over the opening. 

 Although a closed vesicle is thus formed, it is made up of the non- 

 cellular cuticle which extends uninterruptedly over the cupula of the 

 invaginated layer of cells, while the secretory mass elaborated by the 

 sensory cells becomes enclosed to form a vitreous body (the marine 

 polychgete worm. Nereis— Hesse, 1897-1908) (Fig. 101). 



Once this stage has been reached, further advances can be made in 

 the optical arrangements of such an eye. The simplest is the more or 

 less elaborate thickening of the cuticular layer of the epithelium to 

 form a refringent apparatus. In its most primitive form such an eye 

 consists merely of a group of visual cells arranged in a hollow beneath 

 a lens formed from the cuticle as is seen, for example, in the medusoid, 

 Sarsia, or the louse, Pediculus, or other insects (Fig. 106). A somewhat 

 similar morphology is seen in the eye of the Onychophore. Perijmtus,^ 

 but in it the large lens is formed from the hypodermal cells and 

 takes the place of the vitreous (Fig. 103) (Cuenot, 1949). Usually, 

 however. h\^odermal cells continuous, on the one hand, with the 

 surface ectoderm and, on the other, with the sensory cells of the 

 cupula, edge their way underneath the cuticle where they may form a 

 clear, refractile mass underneath the cuticular lens constituting a 

 primitive lens or vitreous (as in the ocelli of many insects and in some 

 spiders. Figs. 104 and 105) (Biitschli, 1921; Wigglesworth, 1941; and 

 others). Alternatively, as in the C'ubomedusan, Charybdea, the distal 

 ends of the retinal cells (rhabdites) develop greatly to form a clear 



1 p. 224. 2 p. 204. 



^^^^a^miD 



Larva of Musca 



Nautilus 



Nereis 



Sarsia 



Pediculus 



Peripatus 



