No. I.] EYES OF MOLLUSCS AND ARTHROPODS. 8/ 



In Branchipus the nuclei of the retkiophorae are situated in 

 protoplasmic thickenings over the outer ends of the rods. A 

 corneal hypodermis is present, composed of indefinitely ar- 

 ranged cells. 



The style is a flattened tube, containing an axial nerve-fibre. 



In Mantis religiosa there are two corneal cells beneath each 

 facet. Each ommatidium has seven retinulae and at least six 

 light-brown pigment-cells surrounding the calyx. There are 

 also two large black cells enclosing the neck of the calyx. 

 Their inner ends terminate abruptly at the outer end of the 

 style. Three of the retinulae are longer and more deeply pig- 

 mented than the others. 



The basal membrane is a thick layer of nucleated connective- 

 tissue, permeated by canals corresponding in number with the 

 ommatidia ; through each canal passes a bundle of pigmented 

 nerve-fibres. 



Just beyond the narrow neck of the calyx the four axial 

 nerves contained in the style break up into four bundles, one 

 entering each chamber of the calyx. There each fibre gives 

 rises to innumerable horizontal fibrillae, which unite with each 

 other to form a complete nervous net-work. 



The style is surrounded by six nerve-fibres, which appear in 

 cross-sections as so many small dots. They may be followed 

 as far as the calyx, where they break up into numerous smaller 

 branches, continuous with those inside the calyx, by means of 

 minute cross fibrillae. The outer ends of the retinulae are re- 

 duced to structureless membranes which unite to form a sheath 

 around the calyx. The abaxial face of each retinula is provided 

 with longitudinal nerve-fibres connected with each other by cir- 

 cular fibrillae. Around the retinulae are several, probably eight, 

 bacilli. The nerve-fibres surrounding each bacillus supply the 

 outer pigmented ends of the same. 



The various stages in the formation of ganglionic cells out of 

 sensory ones may be studied in the mantle edge of Molluscs. 

 They arise in the following manner : The nucleus of a slender 

 sense-hair cell, which terminates inwardly in a long fibre exactly 

 similar to the prolongations of the neuro-epithelial cells of 

 Coelenterates, wanders below the basal membrane (Fig. i8,lll.), 

 while the outer end of the cell is reduced to a fine fibre, still 

 terminating in one or more sense-hairs. 



