igo8.] R. E. Lloyd : Bathynomus giganteus. 95 



which is composed of two separate halves. This separation of the vitrellae into two 

 halves can be seen, with some difficulty, in nearly all sections of those structures, 

 both longitudinal and transverse. The separation is shown in the photograph (fig. 5), 

 in which one of the lenses seen in transverse section is ill developed, and shows the 

 division very clearly. Each vitrella is separated from its neighbours by a thick layer 

 of pigmented protoplasm, which appears to be continuous below with the pigmented 

 retinula cells. This pigmented material not only lies between the vitrellae but spreads 

 out above them, covering the peripheral part of their upper ends, so that only the 

 axial part of each can transmit light. Vertical sections which pass through the 

 central axis of the lenses show that the lower ends of these organs are almost in 

 contact with a continuous pigmented layer, which forms the innermost limit of the 

 perceptive pa,rt of the eye (fig. 4). 



Rhabdomeres appear to be altogether absent. Careful examination of many sec- 

 tions failed to disclose them ; though it is possible that there are small rudiments of 

 these structures among the densely pigmented retinula-cells. 



Around the lower or inner ends of the vitrellae are grouped some four or five 

 (the number was not definitely determined) densely pigmented cells. These are the 

 retinula-cells. They are prolonged at their lower ends into pigmented filaments, 

 which, after a short twisted or spiral course, pass into and make up a continuous 

 pigmented substratum, lying beneath the lenses and forming the nnermost limit of 

 the perceptive part of the eye. One of these pigmented filaments, which were seen 

 on many occasions in vertical sections, is shown in the photograph (fig. 4). In the 

 pigmented substratum these filaments doubtless communicate directly or indirectly 

 with the fibres of the optic nerve. The optic nerve becomes flattened out as it ap- 

 proaches the eye ; it passes on to the inner side of that structure and spreads out in a 

 fan-shaped form. 



Theoretical Considerations. 



The large vitrellae, which are remarkably clear and translucent, the well-developed 

 black pigment, which not only separates these lenses but spreads out and covers 

 them, peripherally like an iris, and the well-developed pigmented terminations of 

 the retinula-cells, all point to the conclusion that the eye of Bathynomus is a useful 

 light-perceiving organ. This fact lends support to "the theory of abyssal light," for 

 Bathynomus is essentially a deep-water form which does not seem to be a recent emi- 

 grant from shallow waters. 



The absence of a rhabdom does not seem to be of much significance, for this organ 

 cannot be considered an essential part of the eye of an Isopod. Thus, among those 

 species which have been examined, the rhabdom varies very much in its degree of 

 development. Beddard (20, 21) found that in Ardurus furcatus, a. deep-sea species, 

 the rhabdom was hardly less than the vitrella in size, while in Serrolis scythei the 

 rhabdom was considerably smaller than the vitrella. Again in a third case Beddard 

 says, " Bullar has not figured or described more than a clear point at the summit of 

 each cell of the retinula in Cymothoa, which he regards as the equivalent of a rhabdo- 

 mere.' ' The presence or absence of a rhabdom has, therefore, no relation to the range 



