168 WILLIAM J. DAKIN 
invagination of the ectoderm. ‘The vesicle cut off gives rise 
to the subcorneal layer and the retina (see fig. 5). The lens is 
secreted within this vesicle and is non-cellular. It has no 
connexion directly with the cuticle of the body-wall, nor is 
the latter thickened as it passes over the cornea. 
The description already given shows clearly that we can 
exclude the complicated compound eyes of the Insects and 
Crustacea so far as our comparison is concerned. No informa- 
tion regarding the origin of the compound eye of the arthropoda 
is likely to be obtained by the study of the Eye of Peripatus. 
Comparison must be made, then, with the lower and more 
TEXT-FIG. 2. 
Insect ocellus (Helophilus) after Hesse, somewhat modified. 
C. =cuticle; C.L, = cutic. lens; Conn. = connective tissue; hy. = 
hypodermis; #.c.=rod-cells of retina, Note difference in 
character of lens from that of Peripatus. The formation of lens 
by thickening of cuticle over eye is very characteristic in Insecta. 
simple arthropod visual organs, the simple eyes. We shall 
also exclude the Arachnoid eyes, the structure of which (see 
Lankester (6), and Watase) is again different in type. We 
are left with the Myriapod eyes and the larval eyes and ocelli 
of insects. 
A marked difference is easily recognized between the Eye 
of Peripatus and the above. In the ocelli of insects (Helo- 
philus, Ceratopsyllus, &c., see Text-fig. 2) and in the larval 
eyes, we usually find that the ectoderm is invaginated 
to form the retina (see literature 2 and 8). We do not 
find a complete vesicle. The ectoderm does not give 
