THE PYCXOGONIDS. 61 



ceding older stage. A narrow zone of pigment cells forms the 

 inner layer of the eye, and at the lower corner of the eye it passes 

 directly into the hypodermis below the eye, while at the upper 

 corner of the eye the pigment layer is never found intimately 

 connected with the hypodermis, but is in close contact with the 

 middle layer of the eye. Fig. 50 cuts the upper part of the eye 

 in the middle line, but not so much so the lower. In the upper 

 corner of the middle layer there lies a cell which is shorter and 

 broader than the retinal cells farther down, and very often the 

 second cell from the top shows a somewhat similar difference in 

 shape. In fact, these two cells seem to be present in all the 

 larval eyes, and the meaning of this we shall see later. 



Fig. 48 shows the posterior eye of a still younger larva. (See 

 Fig. XYI, Plate Y.) Here again we find practically the same 

 conditions as in the preceding figures, but the eye is smaller. 

 The corneal hypodermis passes at the lower corner into the 

 middle layer of the eye. The pigment zone behind the eye is 

 narrow, and the third layer is seen to continue into the hypo- 

 dermis below the eye. At this place the cells of the hypodermis 

 are very distinctly seen passing into the pigment layer of the 

 eye. The middle layer is separated very distinctly from the 

 corneal hypodermis by an elongated cavity, probably caused by 

 shrinkage. 



Between this eye and the next earlier stage there is quite a 

 difference in size and structure. This next stage is shown in 

 Fig. 47, which is from a larva having a single pair of walking 

 legs. (See Fig. XY, Plate Y.) Both posterior eyes are shown 

 in this figure (47). They are not, as in all older eyes, raised above 

 the general surface of the body. Each eye is still seen to be 

 three-layered, though this is not so apparent as in older stages. 

 Both the outer and the inner layers are quite narrow. The 

 middle layer is seen to be formed of two quite large clear cells 

 with very large nuclei. These form by far the most conspicuous 

 part of the eye in cross-section, but I have not determined defi- 

 nitely whether there are only these two cells, as seen in cross- 

 section, or whether there are a few more present. There is, it 

 must be confessed, not much evidence of invagination of the eye 

 at this stage, and for this reason it seemed better to begin with 

 older stages, which could be easily interpreted as such. How- 



