MUSEUM OF COMPARATIVE ZOOLOGY. 63 
of the ommatidia is invariably hexagonal. In the remaining macrurous 
Decapods? 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,? 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 Leuckart, ’47, p. 204, Chatin, ’78, p. 8); 
Callianassa; and Gebbia (Milne-Edwards, ’34, p. 117). In Pagurus longicarpus, 
Say, and Hippa talpoida, 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 (Miiller, 26, Tab. VII. Fig. 18, Leydig, ’57, p. 252, 
Fig. 134, Reichenbach, ’86, Taf. XIV. Fig. 226, Huxley, ’57, p. 353) ; Homarus 
(Newton, ’73, Plate XVI. Fig. 3, Parker, ’90%, p. 8); Palemon (Grenacher, ’79, 
Taf. XI. Fig. 118 A, Patten, ’86, Plate 31, Fig. 115); Penaeus (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 Bartonit, 
and Palcemonetes vulgaris, Say. 
2 According to Chatin ('78, p. 13) the outline of the facet in T'ypton 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 (’86, Plate 31, Fig. 
116), the ommatidia are hexagonally arranged, although it must be borne in mind 
that Will's (’40, Fig. III. c.) figure of the facets in Galathea strigosa affords unmis- 
takable evidence of a tetragonal arrangement. 
