RELATION OF MEDIAN TO LATERAL EYES 355 



stages of development in the embryos of vertebrates, that the pre-verte- 

 brate stock from which the vertebrates arose passed through the same 

 stages in development and had sensory organs and nervous system which 

 was built up on essentially the same plan as that previously mentioned in 

 connection with Planaria. 



The study of the eyes of molluscs emphasizes another question bearing 

 upon the history of the paired lateral eyes of the larva. In some instances 

 in place of further development and differentiation taking place in the 

 course of ontogeny, a reverse process is observed, namely degeneration. 

 Thus in certain Lamellibranchia and in Chiton, cephalic eyes appear 

 temporarily during the development of the larva and later disappear 

 when, having become covered by the shell, they are rendered useless. 

 This disappearance of the larval eyes is found chiefly in those animals 

 which burrow in the mud, live in deep sea water, or are parasitic. In 

 some cases they are replaced by eyes of a different type, which are secon- 

 darily acquired, e.g. the eyes on the edge of the mantle in some bivalve 

 lamellibranchs, such as Pecten (Fig. 106, Chap. 12, p. 148) and on the 

 back of Chiton (Fig. 115, Chap. 12, p. 157). In Chiton some of the 

 megalaesthet.es, or large sensory organs, become transformed into what 

 appear to be eyes. Each of these eyes is covered by a pigmented layer of 

 cells which envelops a modified aesthete ; superficially is an arched layer 

 of the tegmentum which forms a cornea beneath which is a lens, and a 

 cell layer which is regarded as a retina. The individual cells of the retina 

 are connected by nerve fibres with the nerves of the ordinary aesthetes . 



In connection with the degeneration of the larval lateral eyes of 

 Pecten and Chiton, it may be noted that the simple form of eye found in 

 Nautilus (Fig. 112, Chap. 12, p. 152), which consists of merely a spherical 

 optic pit, opening to the exterior by a constricted orifice with neither 

 lens, iris-fold nor cornea, is probably an instance of arrested development 

 of a formerly more highly developed organ rather than the retention of a 

 primitively simple form. It may be presumed that this arrest occurs at 

 an early stage in its formation, and it may be regarded as an inherited 

 condition which originated in association with a change in the habits 

 of the adult animal from a creeping or possibly actively swimming creature 

 to a passive state in which the animal floats about enclosed in the last 

 compartment of its rigid spiral shell (Fig. no, Chap. 12, p. 151). 



The distinction between a simple structure, which is simple because 

 it has retained its original simple form unchanged throughout its ancestral 

 life-history, and one which is simple as the result of degeneration is in 

 some cases very difficult and can only be decided by a very careful con- 

 sideration of the animal as a whole, in all its bearings, structural and 

 functional, embryological and phylogenetic. Unfortunately, as far as 



