446 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



K "1 THK OMMATII'I I M . 



Both a lack of time and of fresh material have prevented me from making as thorough a study 

 of the structure of the oinmatidiura of Alpheus as I had wished. The following account is based 

 entirely 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 in A. hetcrochelis). There are two corueal cells to each lens. A single ommatidium is shown 

 in Fig. 200. As in all Decapods, the cone cells which underlie the corueagen 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 in which the nuclei of the coue cells lie, although it is not always easy to distinguish 

 them. This cap appears 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 retiuular cells (pg. c P). LIV), as Parker (48) designates them, and at least 

 seven proximal retinular cells (rtl.). Parker discovered in Ilomarus 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 Pala> 

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

 domeres. A transverse section of the rhabdom gives the peculiar 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 in dark pigment. 



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

 light and is amber color in transmitted light. This may be similar to the pigment of certain cells 

 which occur beneath the cuticle of the larva of Decapods in various parts of the body. It is not 

 decolorized 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 nitric acid. These cells penetrate the base- 

 ment membrane, 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 number of accessory pigment cells belonging to each 

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

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

 seen widely diffused, while there 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 next the basement 

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

 of the coues, while the cones themselves were draped in black. 



ARRANGEMENT OF THE OMMATIDIA. 



In Alpheus saulcyi the omraatidia are arranged in a hexagonal system, subject to variations 

 in different 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 hexagonal. There is probably considerable individual variation. 1 have examined the 

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

 a West Indian species closely allied to A. heterochelis. These cases afford some very interesting 



* For ;i stmly of the coruea, adults of the largest size were selected and the cuticle was cleaned by boiling in a 

 concentrated solution of potassic hydrate. 



