356 THE PINEAL ORGAN 



we are aware, the ontogenetic development of Nautilus has not yet been 

 studied. The complicated structure of the adult tetrabranch Nautilus 

 and a consideration of this structure in relation with the general principles 

 and facts of comparative anatomy, combined with a study of the various 

 types of extinct Nautilidse, appear to warrant the general conclusion that 

 the mobile shell-less forms with actively functioning eyes come first in 

 the order of evolution, and that fixation or the development of a shell is 

 usually followed by the degeneration of the locomotive organs and of the 

 eyes. At the same time it is almost certain that the eyes of the Nautilidse 

 never attained the high degree of evolution which has been reached in the 

 cuttle-fishes. The shell was developed at a very early period in the 

 phylogenetic history of the Nautilidae, which are pre-eminently Palaeozoic 

 in their distribution, and it seems likely that the truth lies between the 

 two suppositions, and that the eye of Nautilus is simple partly because 

 it has undergone degeneration and partly because it has retained its 

 primarily simple form without having undergone a high degree of 

 specialization. 



Eyes of Mollusca 



Among the various types of eye which are met with in the Mollusca 

 it is worth while noting, with reference to the inverted eyes of Pecten, 

 the singular compound eyes of the upright type which are found round 

 the edge of the mantle of the " Noah's-ark " shell-fish (Area Noce). These 

 appear on the summit of small rounded projections, and consist of a 

 dome-shaped epithelial cap which covers and encloses a central meso- 

 dermal core. In microscopical sections (Fig. 40, Chap. 3, p. 58) the 

 epithelial cap is seen to be composed of a single layer of tall columnar 

 cells, which are arranged in groups or units, each of which is composed 

 of one central visual cell surrounded by six cylindrical pigment cells, 

 thus forming an ommatidium resembling somewhat the ommatidia of 

 certain arthropods. The ommatidia of Area are, however, separated by 

 slender unpigmented interstitial cells. The sensory or visual cells are 

 conical in form, the base of the cone being slightly convex and directed 

 outward, while the apex is directed inwards and rests on a subepithelial 

 limiting membrane. A clear spherical nucleus is present in the super- 

 ficial segment of the cell, whereas the tapering inner part of the cell has 

 a rod-like structure. Although they resemble in certain points the simple 

 forms of aggregate eyes of some arthropods, they are regarded as having 

 been evolved independently and as having no genetic relation with these. 

 Moreover, although there is a marked difference between the mantle 

 eyes of Area and those of Pecten, the single layer of columnar pigment 

 cells which form the iris-like zone which surrounds the cornea and lens 



