64 BULLETIN OF THE 



servations appear to me to afford considerable evidence in favor of the 

 view that the hexagonal arrangement is phylogenetically more primitive 

 than the tetragonal. 



Granting this conclusion, a number of otherwise exceptional observa- 

 tions can be explained. Thus, as long ago as 1840, Will ('-±0, p. 7) 

 called attention to the fact that in Astacus, where the ommatidia are 

 normally arranged . upon the tetragonal plan, facets near the edge of the 

 retina are often irregularly hexagonal. The edge of the retina is well 

 known to be the last part produced, and therefore it is probably the 

 part least differentiated. Admitting the hexagonal arrangement to be 

 a primitive one, it is only natural to expect that, if it persists at all, 

 it will persist in the less modified portion of the retina. Hexagonal 

 facets also occur on the periphery of the retina in Ilomarus, and are to 

 be explained, I believe, in the same way. 



On the assumption that the hexagonal plan is primitive, the occur- 

 rence of a few genera with ommatidia hexagonally arranged, in a group 

 in which the tetragonal arrangement is the rule, can also be explained. 

 InTypton, for instance, the hexagonal plan obtains, although in almost all 

 Crustaceans closely related to it the tetragonal system prevails. This 

 condition may be explained, however, by the fact that the eyes in Typton 

 show evident signs of degeneracy, due in all probability to tJie parasitic 

 habits of the Crustacean. If the hexagonal arrangement represents an 

 early ontogenetic phase in the development of Decapods related to Typ- 

 ton, it would be natural to expect that in Typton itself, where the normal 

 development of the eyes is interrupted by parasitism, this arrangement 

 would persist permanently. 



In Galathea, as I have already mentioned in a note on page 63, the 

 ommatidia according to Will are arranged tetragonally ; according to 

 Patten, hexagonally. At first sight these observations might appear 

 to be irreconcilable, but such is not necessarily the case. So far as I 

 have been able to ascertain, Patten does not mention the name of the 

 species which he studied. Possihly he may have examined some other 

 than G. strisrosa, the one from which Will's figures were drawn. In 

 such an event, a difference in the arrangement of the ommatidia may 

 have been characteristic of the two species, although, if both possessed 

 well developed eyes, this difference would be somewhat anomalous. If 

 this is not the true explanation, it is stdl possible that the specimens 

 studied by Patten were somewhat immature, in which case the hexagonal 

 arrangement might very naturally he present. From what has been said, 

 I think it must be evident that the apparent contradiction in Will's and 



