446 MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



STKILTUEE OV TIIK' OMMATIDIUM. 



Both a lack of time autl of fresli material have pieveuted me from makiug as thorough a study 

 of the structure of the ommatidium of Alpheus as I had wished. The followiug account is based 

 eutirely upon sections : 



The corneal facet is strongly biconvex (Fig. 200), the convexity of the lower side being the 

 greatest. Its shape is usually hexagonal, but may be tetragonal, or sometimes nearly circular 

 (as iu A. heterooheiis). There are two corneal cells to each lens. A single ommatidium is shown 

 in Fig. 200. As in all Decapods, the cone cells which underlie the corneageu are four in number. 

 The four segments of the crystalline cone (Fig. 208), which are secreted on the inner sides of the 

 latter cells, are always separated by delicate boundary lines. The cone is capped by a mass of 

 protoplasm iu which the nuclei of the cone cells lie, although it is not always easy to distinguish 

 them. This cap api)ears to be raised into a slight elevation which touches the center of the lens. 



Pigment cells invest the cone more or less completely according to the conditions under which 

 the eye is examined. These are the retinular cells. In the larva, and probably in the adult also, 

 there are two distal retinular cells {2)g- c PI. Liv), as Parker (48) designates them, and at least 

 seven proximal retinular cells (>-tl.). Parker discovered in Homarus and several allied forms 

 a rudimentary eighth cell belonging to the proximal series. This is present I believe, in Alpheus, 

 although my sections do not show it with the same clearness that it can be demonstrated in Palje- 

 monetes. The seven proximal retinular cells secrete on their inner sides the rhabdom or rhab- 

 domeres. A transverse section of the rhabdom gives the i)eculiar seven-pronged figure shown in 

 the drawing (Fig. 205). The cells appear as fused in section, but possibly they would separate 

 with readiness if macerated. Unfortunately I had no fresh material to experiment with. The 

 proximal retinular cells appear to penetrate the basement membrane, and they are continuous 

 below it with nerve fibrils. As to their distal ends, I have seen no evidence that they extend out 

 to meet the cornea. The retinular cells abound iu dark pigment. 



The accessory pigment cells secrete a peculiar pigment which is glistening 'white in reflected 

 light and is amber color iu transmitted light. This maybe similar to the pigment of certain cells 

 which occur beneath the cuticle of the larva of Decapods in vai'ious parts of the body. It is not 

 dfecolorized when subject to the prolonged action of weak solutions of nitric acid, while the black 

 pigment is completely removed. What I once regarded as chitiuous bodies (20-21) were fused 

 masses of this pigment which had been treated with nitrife acid. These cells penetrate the base- 

 ment membraue, beneath which there is a considerable mass of both yellow and black pigment. 

 The trains of cells which accompany nerve fibrils into the fibrous portions of the optic ganglia also 

 contain granules of black pigment. The uumber of accessory pigment cells belonging to each 

 ommatidium is indeterminate. They have the power of free movement or migration outward from 

 the basemeut membrane and the power of retraction like the retinular cells. In Fig. 200 they are 

 seen widely ditt'used, while tljere is a zone of black pigment cells (the pigment withdrawn by acid) 

 enveloping nearly the entire cone and the distal end of the rhabdom. In an eye taken from a prawn 

 which had just moulted, the yellow pigment was restricted to a narrow zone uext the basement 

 membrane. Outside of this belt the retinular cells were colorless nearly up to the proximal ends 

 of the cones, while the cones themselves were drai)ed iu black. 



AKKANGEMENT OF THE OMMATIDIA. 



In Alplieus saulcyi the ommatidia are arranged in a hexagonal system, subject to variations 

 in difl'erent parts of the eye.* In the central parts of the cornea the facets are symmetrical hexa- 

 gons. On the lower side of the eye the rows are more irregular and individual facets tend to 

 become square and rounded. Toward the outer side of the eye the facets are very nearly square, 

 next to these they become irregular and rounded, and on the extreme outer edge the facets are 

 sometimes hexagoiuil. There is probably considerable individual variation. 1 have examined the 

 cornea iu four other species of Alpheus, namely in Alpheus heterocheUs, A. minor, A. normani, and 

 a West Indian species closely allied to A. heterocheUs. These cases attbrd some very interesting 



* For a stuily of the cornea, ailiilts of the largest size were selected and the cuticle was cleaned by boiling in a, 

 concentrated solution of potassic hydrate. 



