358 THE PINEAL ORGAN 



viscero-parietal ganglion. Each eye is protected by a triangular fold of 

 the epithelium, which forms an overlapping screen. The first stage in 

 the development of these eyes is represented by a depression of the 

 modified central area of epithelium at the apex of an ocular tentacle 

 which forms a hollow cup. Later, by proliferation of the cells at the 

 bottom of the cup, a solid mass of epithelium is formed which becomes 

 differentiated into a superficial part, the lens, and a deeper layer, the 

 retina, one side of the cup becomes raised to form the triangular protective 

 screen, the inner limb of this fold is continuous with the opposite lip 

 which covers the lens and forms the cornea. The retinal portion appears 

 to be invaginated in much the same way as in the formation of the secon- 

 dary optic vesicle in vertebrates. 



This brief summary of the structure and development of the various 

 types of molluscan eyes indicates that the higher orders of Mollusca have 

 deviated very widely from the simple forms which represent the ancestral 

 parent stock ; also that the eyes of different orders of the Mollusca differ 

 very widely not only from each other but from those of other Phyla of 

 the invertebrates and from the vertebrates. The eyes of the cuttle-fish 

 show a high degree of differentiation and have a superficial resemblance 

 to the eyes of vertebrates, but they lack the delicate mechanisms for 

 accommodation of the lens and adaptation of the pupil, and they differ 

 essentially in their mode of development and structure from the verte- 

 brate eyes. Median eyes are only seen in the larval stages of the lower 

 classes of Mollusca ; and the early stages of ontogeny having been abbre- 

 viated or suppressed, they have entirely disappeared in the cephalopods. 

 In the phylogeny of the phylum Mollusca, different types of eye have 

 been evolved in different regions of the body, such as the back and edge 

 of the mantle, by methods which involve folding and invagination of 

 the surface layers ; these are similar to those which occur in other phyla 

 of the invertebrates and in the vertebrates, but have been evolved by 

 processes of parallel development and are not genetically related, and it 

 is only in the case of the paired eyes of the simpler types of molluscs and 

 their trochophore and veliger larvae that the genetic relation with other 

 phyla can be traced with any degree of certainty. 



The Median and Lateral Eyes of Arthropoda 



It is with reference to certain classes of the Arthropoda that the most 

 enthusiastic claims were made for the existence of a close relationship 

 between vertebrates and invertebrates, and in view of the similarity in 

 structure and position of the median eyes of invertebrates and the median 

 or pineal eyes of vertebrates, it was thought that the common origin 

 which was claimed for the Entomostracan and the pineal eyes constituted 



