RELATION OF MEDIAN TO LATERAL EYES 367 



work done by the action of the rays of light involves an expenditure of 

 energy which affects the nerve-endings in the base of the cell. Thus 

 one part of the cell becomes differentiated as a receptor and transmitter 

 of nerve-impulses, and another the pigmented part for the absorption 

 of light. Later special cells or parts of cells are differentiated to form 

 the refractile elements, namely, the vitreous, the cuticular lenses or facets, 

 and the rods and cones ; these lie for the most part superficially and at 

 the clear distal ends of the cells or of the ommatidia. The intracellular 

 pigment or the specialized pigment cells are displaced towards or are 

 formed in the peripheral parts of the cells, or ommatidia ; whereas the 

 neuro-sensory cells tend to sink beneath the level of the surrounding 

 epithelium, and in this way a pit or downgrowth of epithelium is pro- 

 duced from which nerve-fibres pass to a sub-epithelial plexus or join to 

 form an optic nerve. This process of pit-formation or downgrowth of 

 the neuro-sensory cells may occur singly as in the formation of a simple 

 ocellus, or as an aggregate of unit systems within a definite optic area. 

 The unit systems or retinulae may lie close together, being covered by a 

 continuous layer of the cuticle, which in some cases is thickened over 

 each retinula so as to form a corneal lens, as in the lateral eyes of Limulus 

 (Fig. 86, Chap. 11, p. 124), or to a less extent as in the minute plano- 

 convex lenses which are present in the adult lateral eyes of Agelena and 

 Dytiscus marginalis (Fig. 81, Chap. 11, p. 119), or again in the faceted 

 eyes of many insects. In some cases the units may be widely separated 

 from each other, as in the schizochroal type of trilobite eye (Fig. 98), 

 this form being generally regarded as the more primitive as compared 

 with the holochroal forms in which the corneal lenses form a continuous 

 surface. In many cases there lies in front of or superficial to the retinular 

 layer, and between it and the cuticle, a stratum of modified epithelial or 

 " hypodermal " cells. This layer is usually produced by the infolding 

 of the epithelial cells surrounding the mouth of a single visual pit, as in 

 the larval eye of Dytiscus (Fig. 4, Chap. 3, p. 10), or as in the median 

 eyes of Limulus (Fig. 87, Chap. 11, p. 125), and Scorpio (Fig. 251), by a 

 single layer which covers the whole sensory placode. In these cases the 

 retinal layer of the larva is, when first formed, frequently inverted, and 

 the eye consists of three layers, pre-retinal, retinal, and post-retinal, a 

 condition which is somewhat similar to that found in the lateral eyes of 

 vertebrates, but which must not be regarded as foreshadowing the evolu- 

 tion of the lateral vertebrate type, as the two types of eye occur at the 

 ends of two widely divergent stems or phyla of the animal kingdom, and 

 have most probably arisen independently. 



The folding of the epithelial layers which has produced the reversal 

 of the retinal layer must be considered as a mechanical process which 



