MUSEUM OF COMPARATIVE ZOOLOGY. 21 



One can recognize an odd, usually larger retinula, which occupies the 

 lower right-hand corner of each group. The remaining six retinulse 

 are disposed in pairs. In Figure 11, which represents a plane of section 

 deeper than tliat shown in Figure 10, the retinulae, although somewhat 

 reduced in thickness, present nevertheless the same method of group- 

 ing as was pointed out in Figure 10. In this plane one also notices 

 next the odd retinula, a nucleus. This is remarkably constant in its 

 occurrence, both as to position and as to the fact that there is always a 

 single nucleus. When compared with the nuclei of the surrounding retin- 

 ulie, it is found to resemble them very closely. The nuclei of the seven 

 retinulce are characterized by their sharply marked oval outlines, and 

 by the possession of one or two very distinct nucleoli (Fig. 30, nl. px.). 

 In both of these respects the single nucleus agrees so closely with the 

 nuclei of the retinulte, that, were it not for its somewhat smaller size 

 and deeper position, it could not be distinguished from them. The 

 regularity with which it occurs, and its structural peculiarities, incline 

 me to believe that it represents a reduced retinula in which pigment 

 has never been developed. This belief is further supported by the fact, 

 that the additional nucleus is always found next the larger retinula, 

 which from its great size seems to have replaced a second cell. It is 

 therefore probable that each ommatidium of the lobster's eye possesses 

 eight proximal retinula3 rather than seven, and that one of these is 

 rudimentary. 



Below this additional nucleus, the proximal retinul<B pass around the 

 rhabdome. In this region they are deeply pigmented, and so completely 

 envelop the rhabdome that I am of opinion that no appreciable amount 

 of light gains access to it except through the cone-cells. The cone-cells, 

 it will be remembered, extend through the central region of each group 

 of proximal retinula?, until they almost impinge on the distal end of 

 the rhabdome. Thus, by excluding the retinulte, they form a trans- 

 parent shaft, which leads to the distal tip of the rhabdome, and by 

 which that structure can receive light. The rhabdome in transverse 

 section has a four-sided outline. Three sides of the rhabdome are occu- 

 pied each by a pair of retinuljB. These pairs are composed of the same 

 couples which were previously noticed. (Compare Figs. 10 and 14.) 

 The fourth side of the rhabdome is occupied by the seventh or odd 

 retinula. Thus, again, it is noticeable that this retinula occupies a 

 position where, if perfect symmetry were shown, we should expect two 

 retinula?. The rhabdome in transverse section is broadest midway of 

 its length. In this position the retinulse are small, as if closely pressed 



