Eyes of Molluscs and Arthropods. 545 



the centre of the group, and coustitute the axial nerve (PI. 32, 

 ax. n). 



The retinulae beeome modified at ccrtain levels into pigmented 

 swellings for the protection of corresponding parts of the retinophorae ; 

 the remaining parts ave reduced to colorless and hyaline rods, the ba- 

 cilli bc. 



The retinophorae in the Arthropods are expanded at their oiiter 

 ends to form the calyx (fig. 128, c. c.) containing the crystalline cone. 

 The attemiated inner ends are united to form a hollow tube, the style , 

 (fig. 128 st.]; the expansion of the inner end of the style, the rhabdom 

 of Gren ACHER, is called the p e d i c e 1 {pd) . 



The fused, membranous, outer continuations of the inner row of 

 retinulae constitute the sheath of the calyx (fig. 128 rf^). 



The compound Arthropod eye consists of a double layer: a thin 

 outer one, the c o r ne a 1 h y p o d e rm i s , secreting the corneal facets (fig. 

 128 c.hy.)\ and a thick inner, or ommateal layer, consisting of the 

 united ommatidia. 



A reti ne um (fig. 132) is a collection of ommatidia in which the 

 retinidia of both retinulae and retinophorae, or of the latter alone, form a 

 continuous layer, the retinulae retaining their pigment and primitive 

 arrangement around the retinophorae ; e. g. invaginate eyes of all Mol- 

 luscs (except Pecten) . 



An ommateum (figs. 133 and 138) is a group of ommatidia in 

 which the retinidia, produced by the retinophorae alone, are completely 

 isolated; e. g. the compound eyes of Arthropods and Molluscs. Aretina 

 (fig. 140) is composed of a group of ommatidia in which the retinulae 

 have lost their rods and are transformed into pigmentless ganglionic 

 cells; e. g. Pecten and Vertebrates. 



The term ommerythrine, I have applied to the red pigment in 

 all eyes, whether confined to the rods alone, to the retinulae, or to the 

 underlying tapetum. 



Chapter I. Mollusca. 



Area. 



Area Noae^ and in fact all the species of this genus which I have 

 examined, are extremely timid animals, the slightest disturbance being 

 sufficient to cause them to close their Shells for two or three hours. 

 One unconsciously associates with nearly all Molluscs great stupidity 

 and sluggishness ; Iwas, therefore, surprised to see how quickly speci- 



Mittheilnngen a. d. Zoolog. Station zu Neapel. Bd. VI. 37 



