MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 445 



Alpbeus in the only genus in which I have found glands in the eyestalk. These arc most notice- 

 able at the peripheral parts of the stalk between the basement membrane and the ganglia. They 

 are in reality parts of the green gland, which scuds outgrowths from the bases of the second 

 antenna? into the antennules, the eyestalks. the labrum, and the whole front of the head, so as to 

 completely envelope the brain. The histology of the glandular cocca is the .same 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 autennal gland comes a layer of very loose connective tissue. This is 

 specially abundant below the basement membrane of the retina. It forms a continuous sheath 

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



By far the greater mass of tissues of the eyestalk belongs to the optic ganglion. This is 

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

 called " Punct-substanz" or " Ball-substanz," and "substance ponctuee." Viallaues, who has 

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

 terminology (61). He divides the optic ganglion into two parts, an external and an 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 clii- 

 asma (Fig. 178, Cb. Ex.). This, according to Viallaues, corresponds to the center of the corneal 

 surface, and consequently to that of the limiting membrane of the eye. The internal portion 

 between the external chiasma and the optic peduncle is composed of three principal masses of 

 punct-substauz : (1) la masse medullaire externe, (2) la masse inedullaire interne, (3) la masse 

 inedullaire terininale. The external medullary mass is united to the masse medullaire interne by 

 an internal chiasma, while a fibrous peduncle joins the internal medullary mass to the masse 

 inedullaire terininale. The nerve fibers which pass between retina and ganglion, he calls the post, 

 retinal fibers, and designates as " optic nerve" the peduncle by which the optic ganglion is united to 

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

 a nuclear layer (couche a uoyaux), a molecular layer (couche moleculaire), and a cellular layer 

 (couche a cellules ganglion naires). 



The punct-substanz of the optic ganglion is thus divided into four principal masses (Figs. 178, 

 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 nerve of Viallanes and others) ; proximal segment (masse medullaire 

 terminate) ; internal middle segment (masse medullaire interne); e.rtenttil middle segment (masse 

 me'dullaire exterue) ; distal segment (lame gaugliouuaire) ; optic nerve (couche des fibers post- 

 r6tinienues). 



GENERAL STRUCTURE OF THE COMPOUND EVK. 



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

 was designated by Patten as the "corueal hypodermis" and later as the "corneagen" (50, 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 hypodermis of the eyestalk. The 

 ouimateum, or eye proper (including those parts which intervene between the cornea 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 different individuals, and the shape and arrangement of the ommatidia 

 may be very irregular in different parts of the same eye. The ommatidia are differentiated clusters 

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

 cells composing theommatidinm is very uniform in Decapods, Stomatopods, and Schizopods, so far 

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

 dermis, 2; crystalline-cone cells, 4; outer pigmeuted retinular cells, 2 ; inner pigmented retinular 

 cells, 7 (functional) ; Accessory pigmented cells irregularly distributed, both above and below 

 the basement membrane, probably of ectodermic origin. 



* The lame ganglionn:iire is called " Retina ganglion" by Clans, who regards it as tho trur ivt.ina ; '-<l;is :mssere 

 Ganglion opticum" by Carriere, aud " periupticum " by Hickaon. 



