No. I.] EYES OF MOLLUSCS AND ARTHROPODS. 



91 



EXPLANATION OF PLATE III. 



ax.n., axial nerve. 

 be, bacillus. 

 b.m., basal membrane. 

 C.C., corneal cuticula. 

 C.C.C., crystalline-cone cells. 

 c.hy., corneal hypodermis. 

 ex.n., external nerve-fibris. 

 g:c., ganglionic cells. 

 l.ax.^ and l.ax.^ axial-nerve loops. 

 nf., nerve-fibres. 

 n.rf., nuclei of retinophorae. 

 n.r/.^, nucleolated nucleus of retino- 

 phorae. 

 nrf.^, aborted nucleus of retinophorae 

 pd., pedicle. 



f>g., pigment cells. 



pg^~^i first, second, and third circle 

 of pigment cells. 



rf., retinophorje. 



r/I', innermost ends of crystalline 

 cone cells, or retinophorae. 



rk., rod, or crystalline cone. 



rt., retinula. 



rt.^~'^, hyaline continuation of the 

 retinulse. 



5^,styleofthe retinophorae, or crystal- 

 line-cone cells. 



V.I., vitreous cell layer. 



v.b., vitreous body. 



v., crystalline cone, or vitrella. 



1. Ancestral arthropod eye. 



2. Same of larval insect. 



3. Ocellus of Scorpio; only one ommatidium is represented. 



4. Posterior ocellus of spiders. 



5. Diagram of compound eye, to illustrate its origin as a modified ocellus. 



6. One of the isolated ommatidia from the hypodermis of a Mollusc. 



7. An ommatidium from a Molluscan retineum. 



8. Ommatidium from the compound eye of Area or Pectunculus ; the re- 

 tinulee fg- * ^ have lost their rods, as is the case in all the succeeding diagrams, 

 and serve only to protect the rod of the retinophorae, or become transformed 

 into ganglionic cells. 



9. Same, with cross, section from the anterior ocellus of a spider. 

 ID. The same, from the ocellus of Scorpio. 



11. The same, from posterior ocellus of a spider. 



12. The same, from the compound eye of Insects and Crustacea. 



13. Two ommatidia from a vertebrate retina, without the outer ganglionic 

 layers. 



13a. Is a cross-section of the rods. 



14. Diagram of an ommatidium, with the corneal facet and its cells, from a 

 compound arthropod eye. The pedicle, walls of the retinophorae, and the style, 

 have been drawn in red for the sake of clearness ; in all other cases the red 

 indicates nerve-fibres ; x is the refractive division between adjacent facets ; a, 

 that between the halves of each facet ;j, thickening, sometimes present, of the 

 abaxial walls of the calyx; the crystalline cone may be present or absent, but 

 it can never fuse with the facet, as is supposed to be the case in Lamfyris. 



\\a and 14^ are cross-sections through the calyx and middle of the style, re- 

 spectively. 



15. Two retinophorae with their ganglionic cells, from the retina oi Pecien, 

 showing the loops of the axial, and external nerves of the rods, the two 

 nuclei of the retinophorae, and five characteristic forms of ganglionic cells; 

 b.m., basal membrane, or septum, of the eye; x, a nerve-fibre terminating on a 

 small ganglionic cell; z, andy, two methods of nerve endings upon the ceU 

 wall of the retinophorae. 



