MUSEUM OF COMPARATIVE ZOOLOGY. 63 



of the ommatidia is invariably hexagonal. In the remaining macrurous 

 Decapods 1 the ommatidia are grouped on the tetragonal plan. This last 

 statement, however, is not without exceptions, for in Typton, and at 

 times also in Galathea, 2 the hexagonal arrangement appears to prevail. 

 An explanation of these exceptions will be offered in a subsequent 

 paragraph. 



Before attempting this explanation, however, it will be well to gain a 

 precise idea of the relation of the hexagonal and tetragonal methods of 

 arrangement. At first sight, it might appear that these two methods 

 had no definite relations, and were simply characteristic of different 

 Decapods. Such, however, is not the case; for, as the development of 

 the lobster shows, the ommatidia in a single animal can be arranged at 

 first according to one plan, and afterward according to the other. In 

 the lobster the hexagonal arrangement characterizes the earlier stages 

 of development, and is replaced only subsequently by the tetragonal 

 grouping. A similar change also occurs in the spiny lobster. Thus, 

 in Phyllosoma, the larva of either Palinurus or Scyllarus, the hexagonal 

 facets observed by Milne-Edwards ('34, p. 115) afford unquestionable 

 evidence of the hexagonal arrangement at this stage. In the adult con- 

 dition, however, both of Palinurus and of Scyllarus, according to my own 

 observations, the ommatidia are tetragonally grouped. In the common 

 lobster and the spiny lobster, then, the hexagonal arrangement of the 

 early stages is replaced by the tetragonal one in the adult. These ob- 



'34, p. 117, Will, '40, p. 7, Frey und Leuekart, '47, p. 204, Chatin, '78, p. 8); 

 Calllanassa ; and Gebbia (Milne-Edwards, '34, p. 117). In Pagurus longicarpus, 

 Say, and Hippa talpolda, Say, I have observed a hexagonal arrangement of the 

 ommatidia. 



1 Judging from the figures given by various authors, the ommatidia of the fol- 

 lowing genera are characterized by the tetragonal arrangement ■• Galathea (Will, 

 '40, Fig III. c); Astacus (Muller, '26, Tab. VII. Fig. 13, Leydig, '57, p. 252, 

 Fig. 134, Keichenbach, '80, Taf. XIV. Fig. 22G, Huxley, '57, p. 353) ; Homarus 

 (Newton, '73, Plate XVI. Fig. 3, Parker, '90 a , p. 8) ; Pahvmon (Grenadier, '79, 

 Taf. XI Fig. 118 A, Patten, '80, Plate 31, Fig. 115); Penmis (Patten, '86, Plate 

 31, Fig. 75). As my present observations have shown, the tetragonal arrangement 

 is characteristic of the ommatidia in Palinurus Argus, Gray, Cambarus Bartonii, 

 and Pahemonetes vulgaris, Say. 



2 According to Chatin (78, p 13) the outline of the facet in Tgpton is hexagonal. 

 Presumably the arrangement of the ommatidia in this genus is upon the hexagonal 

 plan. In Galathea, according to the figures given by Patten ('80, Plate 31, Fig. 

 110), the ommatidia are hexagonally arranged, although it must be borne in mind 

 that Will's ('40, Fig. Ill e.) figure of the facets in Galathea strigosa affords unmis- 

 takable evidence of a tetragonal arrangement. 



