VII 



MOLLUSCA 



279 



to the exterior, and filled by a clear mass of transparent jelly-like secretion 

 the vitreous body (v). In Murex (Fig. 115, A, 3) a further advance 

 has been made the cavity of the vesicle being now completely shut off 

 from the exterior. The interior of the vesicle is filled with homogeneous 

 vitreous substance but a large rounded portion towards the external 

 surface is marked off from the rest by its greater density so as to form 

 a lens (/). 



Study of the successive stages in the development of the more com- 

 plicated eye of the Cuttlefish (Siphonopoda) provides a corresponding 



B 



R. 



FIG. 115. 



The evolution of the eye in Molluscs as illustrated by comparative anatomy (A) and embryology (B). 

 Ai, Patella ; Aa, Trochus ; AS, Murex. Bi, 2, 3, stages in development of the eye of a Siphonopod. 

 /, Lens ; R, retina ; r, rod ; v, vitreous body. 



record of its evolution. In an early stage the eye is in the form of a 

 simple depression of the epidermis (Fig. 115, B, i); in later stages the 

 opening of this depression becomes gradually narrowed,, and its cavity 

 becomes filled with vitreous secretion (Fig. 115, B ; 2) ; finally the eye 

 forms a completely closed vesicle (Fig. 115, B, 3). In most Siphonopods 

 the eye does not remain in this Murex-like stage but proceeds to further 

 complications which need not be detailed here but which culminate in 

 a degree of complexity similar to that of the eye of the higher vertebrates. 

 It is remarkable that in Nautilus, the most archaic of existing Siphono- 

 pods, the eye remains throughout life in a stage intermediate between 



