234 



Mr. F. E. Beddard on a 



his important memoir * on the Arthropod eye. The vitrellaf 

 in both types consists of two cells, which secrete a round or 

 pear-shaped crystalline cone ; this crystalline cone is evidently 

 composed of two halves closely applied together, each half 

 being formed from a single cell of the vitrella. 



The retinula in both types is seven-celled ; each cell secretes 

 a chitinous refracting rod — the rhabdomere ; these become 

 fused into an axial structure — the rhabdom — in PorcelUo ; in 

 Gymothoa each rhabdomere remains separate and within the 

 retinula-cell of which it is a product. 



I have recently studied the structure of the eye in several 

 species of ^ga and allied genera, 

 and find some notable differences 

 from the types already mentioned 

 as well as from all other Isopods, 

 excepting the genus Serolis. In 

 Scrolls I the retinula differs from 

 that of PorcelUo &c. in being 

 composed of only four cells ; each 

 cell secretes at its upper extremity a 

 chitinous rhabdomere : the rhabdo- 

 meres are more or less completely 

 fused together along their inner faces, 

 but the rhabdom is not imbedded 

 between the retinula-cells ; on the 

 contrary, each of these cells, owing 

 to its peculiar shape, is only in con- 

 tact with the upper part of the 

 rhabdom ; the lower j)ortion is sur- 

 rounded hy two large spherical trans- 

 j>arent cells, which Jit iji closely 

 between the four retinula-cells (see 

 woodcut). These cells are distinctly 

 nucleated (h), the nucleus possessing 

 a well-defined nucleolus. In sections 

 it can be readily seen that the rhab- 

 dom, which at its inferior extremity 

 becomes divided into four separate 

 pieces (corresponding of course to 

 the four rhabdomeres of which it is 



composed), is imbedded in, or at least is entirely surrounded 

 by, the substance of these large clear cells. 



* ' SeLorgan der Arthropoden/ Gottingen, 1879. 



t This term has been introduced by Profs. Lankester and Bourne 

 (Quart. Journ. Micr. Sci. 1883, p. 177). 



\ " Eeport on the Isopoda collected during the Voyage of H.M.S. 

 ' Challenger,' " Zool. Chall. Exp. pt. xxxiii. 



II 



Ommatidium of Serolis 

 Schythei. v, vitrella- 

 cells ; B, retinula-cells ; 

 r, rhabdom ; H, hyaline 

 cells. 



