448 MEMOIRS OF THE NATIONAL ACADEMY OE SCIENCES. 



lu the first larva of both Homarus and Alpheus saulcyi I find that the facets are not ouly hex- 

 agonal but tend also to be slightly rounded. In the larva 11""" long the lenses tend to become 

 square toward the center of the cornea, while at the periphery they are smaller and generally hex- 

 agonal. Occasionally, however (just in what part I did not ascertain), the peripheral facets tend 

 strongly to the tetragonal arrangement. In the next case (larva 15.3""" long) the facets in the 

 outer parts of the eye are small and symmetrical hexagons. Toward the center they become 

 larger, nearly square, and there is considerable interlenticular space. 



In the yearling lobster the readjustment of the corneal lenses from the hexagonal to the tetrag- 

 onal system has been eftected over the greater portion of the eye, but the transition is still ilhis- 

 trated iu a very beautiful manner. The cuticular ingrowth or peninsula, already referred to, seen at 

 the upper surface, is hook-shaped, bending backward. In the open angle iu front the retina is least 

 dittereutiated. The corneal facets in this region are small and mostly hexagonal. Following the 

 lines of facets as they curve outward and backward from this point over the convex surface of the 

 eye, we see illustrated iu a very striking way the passage of the hexagon into the s(juare by the 

 gradual reduction of the opposite sides. The sides which are sacrificed are the third and sixth, 

 counting from the side which lies between two adjoining lenses of the same row. Over a consid- 

 erable area the facets are not quite square and thus tend to alternate with those of adjoining rows. 

 The gradual transitiou to the square is attended by a gradual increase iu size. In the narrow 

 angle behind the peninsula, the area of the hexagonal facets is smaller. In all other parts of the 

 periphery the facets are square up to the very edge, as iu the adult. The corneal lens iu an 8-inch 

 lobster has about twice the area of that of the lobster one year old (length nearly 2 inches). 



After reviewing these details the diflicult question arises: What is the significance of this 

 remarkable change, and how is it effected ? 



Parker (48), who has made a careful study of the arrangement of the ommatulia in difterent 

 Crustaceans, recognizes two plans on which these organs are grouped, the hexagonal and the tetrag- 

 onal. He says that " in the Brachyura, as well as in three families of the Macrura, the Hippidie, 

 Pagurid;Tj, and Thalassinid;e, the arrangement of the ommatidia is invariably hexagonal. Iu the 

 remaining macrurous Decapods the ommatidia are grouped on the tetragonal plan." There are, 

 however, exceptions to the latter statement, some of which are mentioned. He regards the change 

 from the hexagonal to the tetragonal system as apparently due to the increase in size aud the con- 

 sequent crowding of the ommatidia, and reaches the conclusion from the various facts presented, 

 that the hexagonal arrangement is phylogeuetically the oldest. Upon this view we should expect 

 to find the eyes of the most highly differentiated of the Crustacea arranged ou the tetragonal sys- 

 tem, whereas iu poiuts of fact the crabs, who are uotorious for their great activity and keen powers 

 of vision, permanently adopt the hexagonal arrangement. The lower Macrura adopt both methods, 

 and in certain species of Alpheus, as Alpheux minor, which are active in habit and show no trace of 

 degeneracy, the change is begun but not coni])leted iu the same retina. In order of time the hex- 

 agonal prism precedes the square prism and the conditions which determine the permanency of 

 each of these systems in the adult life of the individual are undoubtedly inherited, but they do not 

 appear to have a phylogenetic significance, at least I do not see the way clear to an explanation 

 upon this ground. 



Taking, for example, the lobster and the crab, in each case the larval eye represents, we must 

 believe, the more generalized type, the adult eye the more specialized type. The larval eye of both 

 Macrouran and Brachyourau has the hexagoual facet. We may safely conclude that this Js a 

 lU'imitive arrangement. The adult crab, which has a more highly organized nervous system aud 

 keener senses, retains this primitive ari-angemeut, while the lobster, whose senses are without 

 doubt duller, departs from the type, and in the adult the facets of tlie cornea take on a permanent 

 tetragonal shape. The conditions which we would naturally look for are thus reversed iu these 

 two forms. 



We must assume that the change iu the case of the lobster is a useful variation, that it is in 

 some way subservient to keen vision, otherwise it could never, upon the theory of natural selection, 

 have secured such a permanent characteristic footing. On the other hand we must believe that 

 the hexagoual facet puts the eye of the crab into better harmony with its environment than the 

 square facet would do, for if the eye had not been in harmony with external conditions it must 



