2-2 THE VOYAGE OF H.M.S. CHALLENGEK. 



substance, especially towards the circumference. The nuclei of the pigment cells become 

 conspicuous in teased preparations of the eye depigmented by means of nitric acid. 



At the upper extremity of each retinula cell, and lying upon the inner margin, is a 

 clear chitinous rod, the " rhabdomere " (fig. 18, r) ; the four rhabdomeres are more or less 

 closely united to form the rhabdom, which is shown infigs. 2, 5,r ; the lower extremity of the 

 rhabdom is produced into a fine thread, reaching nearly as far as the pigmented membrane 

 which bounds the inner surface of the eye. Below this membrane, which is of some 

 thickness and pigmented only upon its upper surface, the retinal cells are continued into 

 stout nervous rods which are .slightly swollen at the upper end, where they come into 

 contact with the retinal cells. 



The pigment sheath of the retinal cells is continued for a short distance along the 

 nerve rods ; the latter exhibit transverse markings, and are a little like striated muscular 

 fibres ; it is very likely that these structures in other Crustacea have contributed to the 

 erroneous idea that the Arthropod eye possesses intrinsic muscular fibres serving as a 

 focusing apparatus. It has, however, been clearly shown by several investigators that 

 there are no such muscular fibres present. 



So far the eye of Serolis schythei only differs from that of other Isopoda in unim- 

 portant details. I now proceed to describe another structure which enters into the com- 

 position of the eye in all the species of Serolis that I have examined, but which has not to 

 my knowledge been figured or described as occurring in the eye of any other Arthropod ; 

 this structure consists of two large hyaline bodies situated below the rhabdom, and enclosed 

 by the upper extremities of the four retinal cells (fig. 2, h). Occasionally only one seems to 

 be present in a single retinula, and very often the size of the two is unequal, one being 

 considerably larger than the other (fig. 10). Each of these bodies is clear and transparent, 

 the substance of which it is composed having very much the appearance of chitin, and I 

 was at first inclined to think that the whole structure in all pioljability represented the 

 rhabdom of other Arthropod eyes. Seeing, however, that a rhabdom is present — though 

 rather small and inconspicuous — this comparison cannot hold good, and moreover each of 

 these hyaline bodies shows an irregularly shaped granular mass, deeply stained by carmine 

 and other reagents, which would seem to be a nucleus ; it is evidently therefore an indepen- 

 dent structure and not a product of the retinal cells ; the lower end of the rhabdom is 

 imbedded in these two cells, and the filiform prolongation of the same appears to pass 

 through their substance. The large size and transparency of these hyaline cells seems to 

 indicate that they serve as a dioptric medium. I am not able to say whether these 

 structures represent highly modified retinula cells or intrusive connective tissue cells, 

 inasmuch as 1 have found them already well-developed in the youngest specimens that I 

 have examined. 



In Serolis paradoxa the structure of the eye is in most respects similar, but each of 

 the cells which compose the retinula secretes in addition to the rhabdomere a small highly 



