424 W. H. GASKELL. 



As to the arrangement of the pigment, the nature and 

 symmetry of its arrangement are seen much better in hori- 

 zontal than in transverse sections. In fig. 22, PL XXVIII, 

 which is a magnified portion of fig. 20 b, PI. XXVII, we see 

 how the pigment is arranged in definite, somewhat conical 

 masses, which terminate at a very distinct limiting edge, and 

 are curved round towards the middle line in certain parts of 

 the eye in a very striking and characteristic manner. 



In the specimens where the pigment has been removed we 

 see that the pigment region is made up of a number of lines 

 arranged radially in the same manner as the lines of pigment ; 

 at the base of these lines numbers of nuclei are seen, which 

 follow in their arrangement the shape of the pigmented region. 

 These nuclei are of two kinds^ large and small ; the large ones 

 are situated at the base of the lines, while the smaller ones are 

 scattered about in between the large ones, and extend to the 

 edge of the eye (figs. 26, 27, PI. XXVIII). In the interior 

 of the eye the limit of the pigment layer is still seen, and no 

 nuclei occur between this limit and the row of basal nuclei. 

 We may look upon this radial appearance as due to the 

 presence of elongated cells, the nuclei of which are situated 

 at the base, and form the more or less regular row of large 

 nuclei seen in all the specimens. The appearance is exactly 

 what one would expect in an eye which is no longer functional 

 if these large nuclei belonged to the nerve-end cells of an 

 Arthropodan eye, as described by Lankester and Bourne (21), 

 Grenacher (22), and other observers. The pigment also corre- 

 sponds in position to that always described in connection with 

 the layer of nerve-end cells. 



As to the smaller nuclei, it is possible that they belong to 

 intrusive connective-tissue cells, as described by Lankester 

 and Bourne (21) in the median eye of Limulus. 



2. The Termination of the Nerve-end Cells with 

 their Attached Rhabdites. — The pigmented layer which 

 forms the posterior wall of the eye is separated from the anterior 

 non-pigmented layer by a central cavity, which according to 

 Beard (20), following the suggestion of Spencer (23), contains 



