MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 445 



Alpbeus iu the only genus in which I have I'ound gliinds in the eyestalk. These are most notice- 

 able at the periijhoral parts of the stalk between the basement memhiane and the ganglia. Tliey 

 are in reality i)arts of the green gland, which sends outgrowths from the bases of the second 

 antennie into the antennules, the eyestalks, tlie labrum, and the whole front of the head, so a.s to 

 completely envelope the brain. The histology of the glandular coeca is the san)e in all parts. 

 They consist of a cubical epithelium, composed of very large cells, supported by a basement mem- 

 brane. 



Near the coeca of the antenual gland comes a layer of very loose connective tissue. This is 

 specially abuudaut below the basement membraue of the retina. It forms a continuous sheath 

 for the optic ganglion, and is reflected over the ojitic peduncle and brain. 



By far the greater mass of tissues of the eyestalk belongs to the oi)tic gauglion. This is 

 composed of ganglion cells (Pig. 187), nerve fibers, and the peculiar fibrous tissue variously 

 called " Punct-substanz" or " Ball-substanz," aud "substance ponctuee." Viallanes, wlio has 

 made very careful and detailed studies of the optic ganglia of Arthropods, uses the following 

 terminology (Gl). He divides the optic ganglion into two parts, an external and au internal 

 portion. The surface of the external region is covered by the limiting membrane of the eye. 

 The external is united to the internal parts by a bundle of crossing fibers, the external chi- 

 asma (Fig. 178, Oh. Ex.). This, according to Viallanes, conesponds to the center of the corneal 

 surface, and cousequently to that of the limiting membraue of the eye. The internal portion 

 between the external chiasm.a and the optic peduucle is composed of three principal masses of 

 punct-substanz: (1) la masse meduUaire externe, (2) la masse medullaire interne, (3) la masse 

 meduUaire terminale. The external medullary mass is united to the masse medullaire interne by 

 an internal cliiasma, while a fibrous peduucle joins the internal medullary mass to the masse 

 medullaire tenniuale. The nerve fibers which pass between retina aud ganglion, he calls the post, 

 retinal fibers, aud designates as " optic nerve" the peduucle by which the optic ganglion is united to 

 the brain. The distal mass of punct-substanz is styled lame ganglionnaiie,* which he divides into 

 a nuclear layer (couche a noyaux), a molecular layer (couche moleculaire), aud a cellular layer 

 (couche a cellules gaugliouuaires). 



The punct-substauz of the optic ganglion is thus divided into four principal masses (Figs. 17S, 

 209). Without adhering closely to the rather cumbrous terminology of Viallanes, the parts of the 

 nervous system contained in the eyestalk between the brain and retina may be designated as 

 follows: Optic 'peduncle (optic uerve of Viallanes aud others) ; proximal segment (masse medullaire 

 terminale); internal middle segment (masse medullaire interne); external middle segment (masse 

 medullaire externe) ; distal segment (lame gangliounaire) ; optic nerce (couche des fibers post- 

 r6tinieuues). 



GENERAL STROCTCRE OF TUK COMPOUN1> EYK. 



The transparent cornea is the secreted product of a specialized layer of the hypodermis, which 

 was designated by Patten as the "corneal hyi)odermis" and later as the "corneagen "(.">(», 51). 

 Beneath this lie several strata of dioptic and sensory cells, separated from the ganglion by a base- 

 ment membrane, which is continuous with that of the general hyjjodermis of the eyestalk. The 

 ommateum, or eye proper (including those parts which intervene between the coruea and basal 

 membrane), is formed by the repetition of a highly specialized unit, the eyelet or ommatidium. The 

 size, number, and arrangement of the ommatidia is characteristic of species or genera, but is subject 

 to considerable variation in diftercnt individuals, and the shape and arrangement of tiie ommatidia 

 may be very irregular iu dilfen-ut parts of the same eye. The ommatidia are ditlereutiated clusters 

 of ectoderm cells. There is a single ommatidium for each lens or corneal facet. The number of 

 cells composing tlie ommatidium is very uniform iu Decapods, .Stoniatoi)o(ls, and Schizopods, so far 

 at least, as the most essential cells are concerned. They are aslollows: Cells of corneal hypo- 

 dermis, 2; crystalline-cone cells, 1; outer pigmented reliuular cells, 2 ; inner pigmented retinular 

 cells, 7 (functional); Accessory jiigmented cells — irregularly distributeil, both above and below 

 the basement membrane. i>rol)ably of ectodermic origin. 



• Tlio lame };an>;liomiiiiro is callcil " Ketiiia Raii^tliim '" by ClaiiH, who regards it aa the true ivtiiia : "das untutcre 

 Gaugliuu oiiticum " by Cairiore, aud " porioiiticum '' by HicksuD. 



