fond 
MUSEUM OF COMPARATIVE ZOOLOGY. 79 
mals only in that their concentric lamella are somewhat more distinct. 
The fact that the lens is composed of concentric layers indicates that 
it is secreted, and the resistance which it offers to reagents is weighty 
evidence in favor of its chitinous nature. In my opinion, therefore, the 
lens in Pontella is a chitinous secretion. 
The development of the lens in Pontella is rather peculiar. Appar- 
ently a new lens is formed with each moulting of the general cuticula ; 
at least, in a rather large proportion of the number of individuals exam- 
ined, the lenses were abnormally small, having a diameter of one third 
or even one fourth of that shown in Figure 18. Moreover, in all such in- 
dividuals the superficial cuticula was correspondingly thin and delicate, 
and when the animal was subjected to boiling potash, the segments of 
its body and appendages separated with a readiness never observed in 
specimens with large lenses. There can be no doubt, I believe, that the 
small lenses are always accompanied by thin cuticula, a relation which 
is to be explained by the immature condition of both structures. 
The smaller lenses differ from the larger ones in only one important 
particular besides that of size. They are not in contact with the super- 
ficial cuticula. This relation can be determined better in optical sec- 
tions than in actual ones, for in the latter the position of the lens is 
usually somewhat changed by the resistance which it offers to the knife. 
The centre of the small lens occupies a position relatively the same as 
that of the large lens, the space between the surface of the small lens 
and the external cuticula being filled with a cellular mass. This mass, 
as seen in optical sections, apparently envelops the lens on all sides, 
and is undoubtedly composed of the cells which secrete that structure. 
As the lens increases in size, the cells are probably excluded from the 
region between it and the cuticula, and as they retreat cement the lens 
to the cuticula. Upon the completion of the lens, the cells which have 
shared in producing it probably withdraw slightly from it to form the 
hypodermal thickenings which occur beneath the adjoining cuticula 
(Plate II. Fig. 18, and Plate III. Fig. 29, h’drm.). These thickenings 
are rich in nuclei, and often have delicate strands of protoplasm stretch- 
ing to the surface of the lens (Fig. 18). 
I believe that these facts justify the opinion that the lenses in the 
lateral eyes of Pontella are composed of chitin, that they are produced 
unconnected with the superficial cuticula, and that they are secondarily 
cemented to it. Like the cuticula itself, they are products of the .hy- 
podermis, a new lens being ‘generated in all probability with each new 
formation of cuticula. 
