Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/576

 526

ARACHNIDA

central eye is called an “ ommatceum.” It shows in Scorpio and Limulus a tendency to segregate into minor groups or “ ommatidia.” It is found that in embryological growth the retinal layer of the central eyes forms as a separate pouch, which is pushed in laterally beneath the corneagen layer from the epidermic cell layer. Hence it is in origin double, and conc sists of a true retinal layer and a postretinal layer (Fig. 24, B), though these are not separated by a membrane. Accordingly the diplostichous ommatoeum or soft tissue of the Arachnid’s central eye should strictly be called “ triplostichous,” since the deep layer is itself doubled or folded. The retinal cells of both the lateral and central eyes of Limulus and Scorpio produce cuticular structures on their sides; each such piece is a rhabdomere and a number (five or ten) uniting form a rhabdom (Fig. 26). In the specialized ommatidia of the compound eyes of Crustacea and Hexapods the rhabdom is an important structure.1 It is a Fig. 24, lateral A, early Scorpio B, .diagram the retinal portion of the same which, owing to the infolding, lies between^, the corneagen p -fi p _ £r p cell-layer; r. portion, and pr, the post-retinal or capsular portion or fold; i, cuticular lens, line not Only agree ux lens-forming eaCU Wltn eaCU in re re separating lens from the lens-forming or corneagen cells of the epidermis; n, nerve fibres; Wi, rhab- garc[ their monOStichoUS and diplodomeres. (From Korschelt and Heider.) How the inversion of the nerve-end-cells and their con- » ,,, ,, nexion with the nerve-fibres is to be reconciled with the condition found in the adult, or with that of StlChOUS Structure DUt alSO m the torthe monostichous eye, has not hitherto been explained, mation in both classes of eyes of rhabcomplex of protoplasmic cells, in which the optic nerve domeres and rhabdoms in which the component pieces are terminates. Each such unit is termed an “ ommatidium.” five or a multiple of five (Fig. 26). Whilst each unit of The lateral eyes of Scorpio consist of groups of separate small lenses each with its ommatidium, but they do not form a continuous compound eye as in Limulus. The ommatidium (soft structure beneath the lens-unit of a compound eye) is very simple in both Scorpio and Limulus. It consists of a single layer of cells, continuous with those which secrete the general chitinous covering of the prosoma. The cells of the ommatidium are a good deal larger than the neighbouring common cells of the epidermis. They secrete the knob-like lens (Fig. 22). But they also receive the nerve fibres of the optic nerve. They are at the same time both optic nerve-end cells, that is to say, retina cells and corneagen cells or con. tiss. secretors of the chitinous lens-like cornea. In Limulus (Fig. 23) each ommatidium has a peculiar ganglion cell developed in a central position, whilst the ommatidium of the lateral eyelets of Scorpio shows small intermediate cells between the larger nerve-end cells. The structure of the lateral eye of Limulus was first described by Grenacher, and further and more accurately by Lankester and Bourne (5) and by Watase; that of Scorpio by Lankester and Bourne, who showed that the statements of von Graber were erroneous, and that the lateral eyes of Scorpio have a single cell-layered or “monostichous” ommatidium like that of Limulus. Watase has shown, in a very convincing way, how by deepening the pit-like set of cells beneath a simple lens the more complex ommatidia of the compound eyes of Crustacea and Hexapoda may be derived from such a condition as that presented in the lateral eyes of Limulus and Scorpio. (For details the reader is referred 25.—Section through one of the central eyes of a young Limulus. L, to Watase (11) and to Lankester and Bourne (5).) The Fig. cuticular or corneous lens; hy, epidermic cell-layer; corn., its corneagen portion immediately underlying the lens; ret., retinula cells; nf, nerve fibres; structure of the central eyes of Scorpio and Spiders and con. tiss., connective tissue (mesoblastic skeletal tissue). (After Lankester also of Limulus differs essentially from that of the lateral and Bourne, Q. J. Mic. Sci. 1883.) eyes in having two layers of cells (hence called diplo2 stichous) beneath the lens, separated from one another by a the1 lateral eye of Limulus has a rhabdom of ten pieces See Fig. 11 in the article Arthropoda. membrane (Figs. 24 and 25). The upper layer is the cor2 ten is the prevailing number of retinula cells and rhabdoneagen and secretes the lens, the lower is the retinal layer. meresThough in the lateral eye of Limulus, Watase states that they may be The mass of soft cell-structures beneath a large lens of a as few as nine and as many as eighteen.

the central and lateral eyes respectively. The central eyes are “ simple eyes,” that is to say, have a single lens, and are hence called “ monomeniscons.” The lateral eyes are in Limulus “ compound eyes,” that is to say, consist of many lenses placed close together; beneath each lens is a