MUSEUM OF COMPARATIVE ZOOLOGY. 61 



arrangement. Thus, hexagonal facets have never been observed except 

 in connection with the hexagonal plan of arrangement. Circular facets 

 are likewise known to occur only with this method of grouping. Square 

 facets, on the other hand, may accompany either the hexagonal or te- 

 tragonal arrangement of deeper parts. 



The hexagonal arrangement is apparently characteristic of the om- 

 matidia in all Crustaceans, 1 except the Decapods. In the Decapods, as 

 will be shown presently, the ommatidia are arranged either upon the hex- 

 agonal or the tetragonal plan. Before proceeding, however, to a descrip- 

 tion of the arrangement of the ommatidia in Decapods, it would be well 

 perhaps to call attention to the rather peculiar grouping of these struc- 

 tures in Gonodactylus, a Stomatopod. 



For a clear understanding of the arrangement of the ommatidia in 

 this Crustacean, it is necessary to have some previous knowledge of the 

 shape of its optic stalk. In Gonodactylus the stalks are elongated cyl- 

 inders, the distal ends of which are rounded. In alcoholic specimens 

 the stalks in an undisturbed position rest with their longitudinal axes 

 approximately parallel with the chief axis of the animal, and with their 

 distal ends directed forward. The retina occupies the free end of the 

 stalk. Dorsally it extends over the distal half, ventrally over only the 

 distal third of the stalk. 



The ommatidia in Gonodactylus are of two kinds, large and small, 

 which are always easily distinguishable from each other, although they 

 differ in no essential respect except size. The large ommatidia are defi- 

 nitely arranged in six rows, which extend as well defined bands from 

 the dorsal posterior edge of the retina anteriorly over its rounded distal 

 end, and posteriorly over its ventral surface to its ventral posterior edge. 

 This band thus occupies both dorsally and ventrally the median portion 



1 Judging from the figures as well as the statements marie by the authors 

 quoted, the hexagonal arrangement is characteristic of the ommatidia in the fol- 

 lowing Crustaceans (exclusive of the Decapods) : Branchipus (Burmeister, '35, 

 p. 531, Spangenberg, '75, p. 30), Neballa (Claus. '89, Taf. X. Fig. 10). Gammartis 

 (Sars, '67, p. 62), Orchestia (Frey und Leuckart, '47, p. 204), Phronima (Claus, '79, 

 Taf. VI. Fig. 48), Cymothoa (Miiller, '29, Tab III. Figs. 5, 6, Bullar, '79, p 514), 

 Lygidium (Lereboullet, '43, p. 107, Planche 4, Fig. 2 b ), Serdis (Owen, '43, p. 174), 

 Arcturus (Beddard, '90, Plate XXXI. Fig. 4), Aneetis (Hesse, '58, pp. 100 and 103, 

 Dohrn, '70, Taf. VIII. Figs. 33, 34), Stpnlla (Milne-Edwards, '34, p. 117, Will, '40, 

 p. 7, Frey und Leuckart, '47, p. 204, Leydig, '55, p. 411), and Mysis (Sars, '07, 

 Planche III. Fig. 7, Grenadier, '79, Taf. X. Fig. 112). I have observed the hexag- 

 onal arrangement in the following genera: Aims. Branchipus, Estheria, Evadne, 

 Argulus, Gammarus, Caprella, Talorchestia, Idotea, Serolis, Porcellio, Sphceroma, 

 Mysis, and Gonodactylus. 



