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XIX. — On the Minute Structure of the Eye in certain Cymothoidce. By Frank 

 E. Beddard, M.A., F.E.S.E., F.Z.S., Prosector to the Zoological Society, 

 and Lecturer on Biology at Guy's Hospital. (Plate XXX.) 



(Read 15th July 1887.) 



The structure of the eye in Isopods has been less studied than in any other 

 group of Arthropods ; the only modern descriptions of the minute anatomy of 

 the eye in these crustaceans known to me are by Grenacher* of Porcettio, by 

 BuLLARt of Cymothoa, by Belloncij of Sphceroma, by myself § of the genus 

 Serolis. I have lately had the opportunity, while engaged upon my Keport upon 

 the " Challenger " Isopoda, of investigating the eye in several species of JEga 

 and allied genera. The structure of the eye in these Isopods differs very 

 materially from the descriptions given by Grenacher and Bullar, but agrees 

 very closely with the structure of the eye in Serolis. 



In all the types which I have studied there is a very close agreement in 

 the structure of the different parts of the eye. . 



To each element of the eye (ommatidium) there is a cuticular lens. The 

 vitreous body is clear and of a yellowish colour ; it is plainly divided into two 

 halves in correspondence with its origin from two cells. In no particular does 

 the vitreous body depart from the characteristic structure exhibited in Serolis, 

 Sphceroma, and other Isopods. 



Underneath the mtrella is placed the retinula. This consists of seven 

 distinct cells, which are deeply pigmented; the retinula cells commonly project 

 upwards nearly as far as the cornea, and thus form a sheath for the vitreous body, 

 as is the case, for example, in Porcellio ; the portion of the retinula cells which 

 borders upon the vitreous body is flattened out and of a greater width than 

 the portion which lies below (fig. 1.6). 



In the possession of seven retinula cells, the Cymothoidse differ from the 

 Serolidae, where there are only four. Bullar has figured seven retinula cells in 

 Cymothoa, and this number may probably, therefore, be regarded as typical of 

 the family. Porcellio and Ligia agree in the number of cells in the retinula. 



Each retinula cell bears on its inner side, as in other Isopods and Crustacea 



* Sehorgan d. Artkropoden, Gbttingen. f Phil. Trans., 1878. 



| Atti. r. Acad. Lincei, 1879. 



§ Eeport on the Isopoda collected during the voyage of H.M.S. " Challenger," Zool., Chall. Exp., 

 Pt. xxiii. 



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