272 ANDREWS. [Vol. V. 



The branchiae are marked by transverse bands of dark red pig- 

 ment, not extending on to the outer side of the main stems 

 where the eyes are ; yet these frequently lie in the general 

 areas of such pigmented bands. 



Each eye is a convex, hemispherical protuberance upon the 

 outer or convex side of the main branchial stem ; not, however, 

 upon the median line, but to one side, as best seen in transverse 

 sections (Fig. 14). The diameter of an eye is about 92 /*., but 

 varies much, smaller eyes being often found toward the tip, or 

 even interpolated between larger ones along the branchia. The 

 color is uniformly dark red, as usually seen, but in strong sun- 

 light (Fig. 1) the reflected light is of a golden yellow color over 

 the surface. Here, also, it is seen that, though the surface is 

 uniformly convex and homogeneous, yet when the microscope 

 is focussed a little beneath the surface, there are conspicuous 

 rounded areas of dark red scattered irregularly through the 

 reflecting pigment, and each appearing like a limpid, liquid 

 mass, transmitting red light from below. These small, clear 

 areas represent what has been known as the component ele- 

 ments of the so-called compound eye, and which I will speak 

 of as the- refracting cells. 



The histological structure of these eyes was first studied by 

 maceration and disassociation. Macerated in Bela Haller's 

 liquid, the eye is readily broken up into a number of elements 

 roughly corresponding to the above dark red areas of the sur- 

 face preparation. Each element has the form of an elongated 

 cone, the base peripheral in the eye, the prolonged apex central ; 

 each is a clear mass covered over by red pigment, and having 

 its basal part easily separable from the surrounding pigment. 

 In potassium bichromate macerations the elements are isolable 

 as large, conical cells (Fig. 2) covered by red pigment granules, 

 but showing the clear, axial portions in places, shimmering through 

 the thin pigment envelope. This clear, axial part of the cell 

 stains with methyl green, as does also a long process extending 

 from the apex of the cell, the central end in the eye, and cov- 

 ered by separate red pigment granules. Some of these cells 

 have bluntly rounded bases, as in the figure ; others, obliquely 

 pointed bases with no pigment over them. In addition to these 

 large cells, refracting cells, there are also much more slender 

 cells (Fig. 3), having a clear, nucleus-like body within the thick 



