MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 447 



facts iu connection with the arrangement of ooimatidia. In Alpbeus normaiii the facets are jren- 

 eraliy symmetrical hexaj^ons, two of the sides beinj,' (|uite short. These tend to run into sqnaies 

 or rounded areas at parts of the periphery. Alplicus minor has the facets in tiie lower part of the 

 eye either of the hexagonal or some polygonal form, while in the upper and outer parts the facets 

 are perfect squares. These blend on all sides into symmetrical hexagons. The first transition 

 from the hexagon to the s(iuare is seen on the outer edges of this area where the individual facets 

 become more and more rhomboidal, as the two opposite sides of the liexagon are more and more 

 reduced. Finally these sides disai)pear and the facets become rectangular. The square facets of 

 any given row now lie opposite to those of the adjoining rows. Four facets belonging to any two 

 rows meet at a common point. In passing, however, from the area of scjuare facets to the periph- 

 eral parts, the facets, though square, are out of line. Starting from a jioint where the facets of a 

 given row are both tetragonal and opposite those of adjoining rows, and following the line over 

 the hemispherical surface toward the perii)hery, the facets of this row soon tend to become alter- 

 nate with those of the row next to it. A facet of the .second row lies slightly in front of the cor- 

 responding facet of the first row. The facet of the third row lies a little iu advance of this and so 

 on until the hexagonal shape is gradually assumed. 



In the Bahaman variety of Alpheun heterochelix the facets in the larval eye are markedly hex- 

 agonal. In the adult there is the same curious transition from the hexagon to the square as we 

 have noticed in Alpheus minor, only it is here, much more striking. In the peripheral parts of the 

 eye, especially in the lower and inner jiortions, the facets are generally hexagonal. In the upper 

 half of the eye between the center and periphery there is a small area of square facets. In Alplieus 

 heterochel is the facts are characterized by much greater looseness of arrangement, there being 

 large interlenticular si)aces. The facets are squares with rounded corners. In some parts of the 

 eye, as on the inner lower side, the facets of the adjoining rows lie o|)posite each other. In other 

 parts they regularly alternate and show a tendency to become hexagonal. At the periphery the 

 facets are widely separated and circular.* 



The markings on the corneal lenses generally consist of a small central spot which is slightly 

 elongated and strongly refracts the light. It ai)pears in section to represent a .slight depression. 

 In the lobster, as first shown by Parker, there is a faint diagonal band which intersects a central 

 hazy spot and divides the scpiare into equal triangles. These diagonal bands are all parallel in 

 adjoining rows. In Alphem sanlvyi the elongated spots, if continued across the lens, would form 

 a series of similar diagonal lines, but none such as these could be detected. In Alplieus heterochelis 

 the markings are very constant and peculiar. In the center of the lens there is a large irregular 

 impression from which numerous rays extend on all sides. The latter do not always reach the 

 periphery. They appear like grooves in the substance of the lens. In the older embryo (Fig. 1!)4) 

 the apex of the cone cells seems to touch the under side of the lens in certain ])arts of the eye. It 

 is therefore possible that these cells remain in contact with the cornea and the interference thus 

 caused gives rise to the spot. A similar explanation is offered by Parker to account for the con- 

 ditions found in the lobster. The significance of the radiating lines seen in Alphcus heterochelis I 

 have not determined. 



Iu Homarus the facets are square and grouped with remarkable regularity up to the very 

 edge of the cornea, excepting at a point on the upper side, wbere a peninsula of tough cuticle juts 

 in from the surface of the stalk and interrupts the elliptical jet black area of the surface of the 

 eye. About this process, particularly iu front of it, the facets are hexagonal or irregular and very 

 much smaller. 



Parker (4S) states that in the lobster " the ommatidia rearrange themselves between the times 

 when the young auimal is 1 inch and 8 inches long. During this period the ommatidia increase 

 about ten times in length and about five times iu breadth.'' I find that this rearrangement begins 

 at a much earlier period, iu fact in the older larval stages. My examination comprised the follow- 

 ing stages : (1) Length 8-9'""' (first larval stage) ; (2) length 11""" (fourth larva) ; (3) length 15.3'"'" 

 (sixth larval stage) ; (4) length 49'"'" (lobster 1 year old). ~ 



*A similar transition of the sqnaro into the hexagonal facet in the same eye occuru iu AstucuH. See Howub' 

 Biological Atlas, Fig. 111. 



