340 ON ISOPODA COLLECTED BY H.M.S. 'CHALLENGER.' [May 20, 



bundle clothed with pigment-cells passes backwards. Two large 

 nuclei of Semper are present, lying between the anterior end of the 

 vitreous body and the cornea. 



In Serolis bromleyana the eye has much the same structure as in 

 Serolis necsra, but the cornea is not faceted and there is no pigment 

 whatever surrounding the vitreous bodies ; the eyes in this species 

 are therefore more rudimentary. 



In Serolis gracilis one of the specimens has eyes exactly re- 

 sembling those of Serolis bromleyana ; in another the whitish tissue 

 of the eye only occupies a comparatively small portion of the whole 

 ocular protuberance; the third specimen is quite blind — the ocular 

 protuberance is present but it is quite opaque and bluish-coloured, 

 like the surrounding integument. In this species therefore the eye 

 appears to be just on the verge of disappearance. 



The only remaining deep-sea form, Serolis antarctica, is entirely 

 deprived of eyes ; two minute yellowish tubercles occupy the position 

 that the eyes ought to occupy, but there is no trace of any cornea, 

 the integument being precisely similar to that on the rest of the 

 body though perhaps a little thinner, and the interior of the tubercle 

 was filled simply with a plug of connective tissue. 



In the species from shallow water the eyes are invariably well 

 developed, and show a general resemblance in structure to the eyes 

 of other Isopoda, but at the same time present certain peculiarities : 

 the retinula is made up of only four cells, which is an unusually small 

 number, five or seven being most commonly met with ; each of these 

 cells is somewhat hatchet-shaped, and the anterior ends are closely 

 applied together beneath the vitreous body ; the lower portion of 

 the retinal cell is elongated, and at about the middle is an oval 

 thickening where the nucleus is situated. In Serolis paradoxa at 

 least each retinal cell secretes a highly refractive body (phaosphere) 

 which may be placed in front of, behind, or to one side of the nucleus. 

 Each of the retinal cells secretes in addition a delicate chitinous rod, 

 the rhabdomere or sehstabchen ; the four rhabdomeres are closely 

 united to form the rhabdom, which differs in structure in different 

 types. In S. schytJiei, S. paradoxa, and S. latifrons the rhabdom is a 

 comparatively small conical body terminating below in a fine thread, 

 which is prolonged backwards nearly as far as the pigmented mem- 

 brane which bounds the posterior surface of the eye. In Serolis 

 cornuta the rhabdom is more complicated, and has much the appear- 

 ance of one of the Malpighian tufts of the kidney, from the ar- 

 rangement of the chitinous rods which compose it into an irregular 

 coil massed round a central piece ; the interstices between the rods 

 of which it is composed are filled with pigment, and the central piece 

 is prolonged into a fine thread. 



So far the eye of Serolis only differs in detail from that of other 

 Isopoda, but there is another structure present, which does not appear 

 to have been described in any other Arthropod, and certainly does not 

 exist in any that I have examined myself. Between the retinal cells, 

 and close to their upper extremity, are two large hyaline bodies 

 nearly as large as the vitreous body, and presenting much the same 



