THE EYE IN CERTAIN CYMOTHOID^E. 445 



Isopoda, and indeed from other Crustacea whose eyes are built upon the same 

 plan, and of which we have any adequate knowledge. So far, therefore, 

 the facts described in the present paper are a further confirmation of the 

 justice of Milne-Edwards' view respecting the affinities of the Serolida?. I 

 would remark that other cases are known where the structure of the eye has 

 been found to indicate affinities, — notably Professors Lankester and Bourne's 

 results in comparing the eyes of Limulus and Scorpio* At the time when 

 I wrote my account of the structure of the eye in Serolis, I was unable to do 

 more than recall the facts, and dwell upon the differences from the eyes of 

 other Arthropods. 



Since that time an important paper has appeared upon the eyes of Molluscs 

 and Arthropods, which has suggested to me an explanation of the apparently 

 anomalous structure of the eye in the Serolidae t and Cymothoidee. 



The author of this paper, Dr Patten, is entirely at variance with Grenacher 

 as to the morphology of the Arthropod compound eye. 



Grenacher regards the ommateum as composed of two layers of cells ; the 

 outer layer, which is grouped into pairs or groups of four, secretes the cuticular 

 lenses, as well as the crystalline cone or vitreous body. Lankester and Bourne 

 have appropriately termed each group a mtrella, to correspond with the term 

 retinula, to be referred to immediately. The inner layer of cells is again 

 grouped into fours or sevens, &c, each group being termed a retinula; each cell 

 of the retinula secretes a hard, chitinous body, the rhabdomere ; the rhab- 

 domeres usually (but not always) unite to form a centrally-placed rhabdom. 

 The retinula cells are prolonged into nerve filaments. Pigment cells are also 

 commonly found isolating the adjacent vitrellae and retinulse. 



Patten agrees with Grenacher in regarding the ommateum as composed 

 of two layers of cells, but these layers do not correspond with those of 

 Grenacher. 



The outermost layer consists of a flattened epidermis, but little modified, 

 which is the matrix of the cuticular facets. Below this come four clear cells, 

 termed retinophorse ; these secrete and enclose an axial, chitinous structure, 

 which corresponds superiorly to the crystalline lens and inferiorly to the 

 rhabdom of Grenacher ; the retinophorse are surrounded and isolated by a 

 variable number of pigment cells, among which the retinulae of Grenacher 

 form one series. The nerve passes up the axial rod and branches into a 

 plexus at its upper extremity. 



If Dr Grenacher's conclusions are to be accepted, the retinula cells and the 

 rhabdom (striated spindle) in the Decapod eye correspond absolutely to the 

 retinula cells and rhabdom in the Isopoda ; the chief difference between the two 



* Quart. Jour. Micr. Sci., vol. xxiii. (1883). 



f Mittk. a. d. Zool. Stat, zu Neapel, Bd. vi. (1886), p. 542. 



