198 PEOFESSOR E. RAY LANKESTER AND A. G. BOURNE. 



should be called a " vitrella." Just as the retinulse develop 

 axial hard structures called " rhabdoms " by Grenacher, so do 

 the vitrellse in varying degree develop dense hyaline bodies 

 within them known as "crystal-cones/' 



The preceding sketch of the degrees of complication of 

 structure presented by Arthropod eyes which the reader will 

 find more readily intelligible if reference be made to the plates 

 of Grenacher's large work, serves to enable us to estimate cor- 

 rectly the morphological significance of the two kinds of eye 

 present in the Scorpions. 



In the lateral eyes we have an example of the simplest kind 

 of Arthropod eye, the monostichous monomeniscous eye. It 

 may be compared with the simplest eye of this kind studied by 

 Grenacher, namely, that of the larva of an Insect (Dytiscus). 

 The woodcut reproduces diagrammatically one of Grenadier's 



¥iG. 1. — Eye of larva of Dytiscus, with monostichous, non-retinulate apo- 

 static ommateura. I. Lens. g. Perineural cells (rudimentary vitreous body). 

 p. Pigment cells, r. Nerve-end cells, o. Filaments of the optic nerve. 



figures of this eye. As in the Scorpion's lateral eye, we find a 

 single row of cell elements continuous with the hypodermis, of 

 which the lateral members (;;. g) are narrow and columnar, 

 whilst the more median are connected with nerve filaments and 

 differentiated as nerve-end cells (r). 



As compared with the eye of the larval Dytiscus, the lateral 

 eye of the Scorpion is in one respect more primitive, in other 

 respects more elaborate. It is more primitive in this, viz. that 

 the row of cells forming the ommateum. is in continuous con- 

 tact with the cuticular lens, whereas in the larval Dytiscus eye 



