PACKAED.] MORPHOLOGY OF PHTLLOPODA. 381 



fragmentary and superiScial. The eyes are closely approximate, ap- 

 pearing as a single eye. Unlike that of Estlieria the number of lenses 

 is small. Plate II, fig. 6, represents the eye of Limnetis gouldii, a circle 

 of crystalline lenses surrounding the central pigment mass or retina. 

 The lenses are contiguous, the pigment not extending between them as 

 in Artemia, Plate XXHI, fig. 6. Plate II, fig. 5, represents the two 

 optic nerves and optic ganglia, the cornea with the crystalline lenses 

 and pigment layer having been torn off with the needle. The optic 

 nerves are very thick, and instead of, as usual, being composed solely 

 of nerve fibers, appear to be largely made up of nerve cells ; fig. 5 a 

 representsan enlarged view from near the middle of the optic nerve, which 

 is made up almost wholly of strings of nerve cells. Toward the distal 

 ends of each optic nerve converge delicate fibers which connect the cells 

 of the optic ganglion with the optic nerves. An enlarged view of the 

 optic ganglion is seen in Plate XXIX, fig. !>. The ganglion cells are not 

 very numerous nor crowded; they are nucleated and nucleolated, and a 

 nerve-fiber broad, triangular next to the cell, rapidly diminishes in size 

 towards the middle of the fiber. It is interesting to notice the inter- 

 communication in the median line of the head between the two eyes; a 

 small number of cells on the opposing edges of each eye are seen to send 

 transverse nerve-fibers (fig. 9, tr. n.) across to the opposite optic gang- 

 lion; though externally the system of crystalline lenses do not quite 

 touch each other. We have not examined the crystalline lenses of 

 Limnetis. 



The eye of Estlieria is nearly on the same general plan as in Limnetis. 

 Lenz has discovered that the lenses are composed of five segments, in- 

 stead of two, the usual number in Crustacea, particularly A2)us and 

 Brancliipiis. 



The inner structure of the eye of Artemia was studied on the living 

 specimens, Plate XXIII, figs. 1 and 6. Fig. 1 shows the general relation 

 of the sessile square simple eye and of the stalked compound eyes to the 

 head and also to the brain. Fig. 6 represents the relations of the eye 

 and its optic lobe to the eye-stalk of the living animal. The optic nerve 

 is in the center; the large rectus muscle of the eye is situated on the 

 hinder or outer side of the stalk, arising near the brain and being in- 

 serted on the cornea at the base of the eye near the first crystalline lens; 

 the exact mode of insertion was not observed. The blood circulates 

 freely, flowing from the head along the anterior side of the outstretched 

 eye, the corpuscles, of different sizes and not very numerous, x)assing be- 

 tween the 0]3tic nervules (op. n.) and returning, as the arrows indicate, 

 along each side of the rectus muscle back to the head. 



The general structure of the eye of Artemia gracilis is much as repre- 

 sented by Leydig in Branchipus stagnalis ; the ganglion opticum, how- 

 ever, is in our specimen of Artemia comijosed of but a single mass, not 

 of two distinct masses connected by coarse nerve-fibers. From the 

 ganglion opticum about a dozen optic nervules penetrate the retina, which 

 is larger in j)roportion to the eye than represented in Leydig's figure; 

 the superficial circle of crystalline lenses or cones showing very plainly. 



The question as to whether the eyes of Crustacea, particularly the 

 stalked eyes, are homologous with the other appendages, and thus rep- 

 resent distinct segments of the head, and which is still held by some 

 naturalists, may, it seems to us, be set at rest by examining the eyes of 

 Phyllopod Crustacea. In the Limnadiadce and Apodidw, where the eyes 

 are sessile, it is easy to see, particularly in Limnetis and Limnadia, that 

 the eyes are modified epidermal cells covering the ends of the optic 

 nerves. They are situated on the front or upper walls or tergum of the 



