MUSEUM OF COMPARATIVE ZOOLOGY. 79 



mals only in that their concentric lamellse are somewhat more distinct. 

 The fact that the lens is composed of concentric layers indicates that 

 it is secreted, and the resistance which it ofters to reagents is M'eighty 

 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 coirespondingly 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 ai*e 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 

 (IPlate II. Fig. 18, and Plate III. Fig. 29, Wdrm.). 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 h}-- 

 podermis, a new lens being generated in all probability with each new 

 formation of cuticula. 



