Fig. [102].—Euphausia pellucida, female, × 5. G, Last gill; L, luminous organ of first leg; L′, luminous organ of 2nd abdominal segment; T, biramous thoracic appendages. (After Sars.)
Fig. [103].—A, Sections (diagrammatic) of Crustacean compound eye, A, with pigment in light-position for mosaic vision; B, with pigment in dark-position for refractive vision. c, Corneal lens; c.g, corneagen cells; cr, crystalline cone; f, basal membrane, or membrana fenestrata; ip, irido-pigment; n, nerve; r, retinula; rh, rhabdom; rp, retino-pigment; v, vitrella.
The compound eyes of Crustacea resemble those of Insects in that they are composed of a very large number of similar elements or “ommatidia,” more or less isolated from one another by pigment. Each ommatidium consists typically of a corneal lens (Fig. [103], c), secreted by flat corneagen cells (c.g) below; beneath the corneal lens is a transparent refractive body called the “crystalline cone” (cr), which is produced by a number of cells surrounding it called the “vitrellae” (v). Below the crystalline cone comes the “rhabdom” (rh), produced and nourished by “retinulacells” (r). The rhabdom is a transversely striated rod, constituting the true sensory part of each ommatidium, and is in connexion at its lower end with a nerve-fibre (n), passing to the optic ganglion. The rhabdoms rest upon a membrane (f) called the “membrana fenestrata.” Each ommatidium is isolated from its fellows which surround it by a complete cylinder of pigment, part of which is especially crowded round the crystalline cone, and is known as “irido-pigment” (ip), while the part which surrounds the rhabdom is called “retino-pigment” (rp).
When the pigment is arranged in this way, as in Fig. A, only those rays of light which strike an ommatidium approximately at right angles to the corneal surface can be perceived, since only these can reach the top of the rhabdom; the others pass through the crystalline cones obliquely, and are absorbed by the cylinder of pigment surrounding each ommatidium, so that they neither reach the rhabdom of the ommatidium which they originally entered, nor can they penetrate to the rhabdom of neighbouring ommatidia. This gives rise to what is known as “mosaic vision,” that is to say, each ommatidium only perceives the rays of light which are parallel to its long axis, and in this way an image is built up of which the various points are perceived side by side by means of separate eye-elements. The distinctness and efficiency of this mode of vision depends chiefly upon the number of ommatidia present, and the completeness with which they are isolated from one another by the pigment. Now this form of vision, depending as it does upon the absorption of a great number of the light-rays by pigment, and the transmission of only a limited number to the sensory surface, is only possible when there is a strong light, and there is no need for economising the light-rays. The most important discovery was made by Exner,[[117]] that the majority of animals with compound eyes had the power of so arranging the pigment in their eyes as to enable them to see in two ways. In bright light the pigment is situated as in Fig. [103], A, so as completely to isolate the rhabdoms from one another (day-position); but in the dusk the pigment actively migrates, the irido-pigment passing to the surface (B) near the tops of the crystalline cones, and the retino-pigment passing interiorly to rest on the membrana fenestrata at the bases of the rhabdoms (night-position). When this happens the rays of light which strike the ommatidia at all sorts of angles, instead of being largely absorbed by the pigment, are refracted by the crystalline cones and distributed over the tops of the rhabdoms, passing freely from one ommatidium to another. In this way the eye acts on this occasion, not by mosaic vision, but on the principle of refraction, as in the Vertebrate eye. Of course the distinctness of vision is lost, but an immense economy in the use of light-rays is effected, and the creature can perceive objects and movements dimly in the dusk which by mosaic vision it could not see at all. The pigment is contained in living cells or chromatophores, and it is carried about by the active amoeboid movements of these cells with great rapidity.
Now, besides the active adaptability to different degrees of light brought about in the individual by these means, we find Crustacea living under special conditions in which the eyes are permanently modified for seeing in the dusk, and this naturally occurs in many deep-sea forms.
Doflein[[118]] has examined the eyes of a great number of deep-sea Brachyura dredged by the Valdivia Expedition, and as the result of this investigation he states that the eyes of deep-sea Brachyura are never composed of so many ommatidia, nor are they so deeply pigmented as those of littoral or shallow water forms. At the same time an immense range of variation occurs among deep-sea forms which are apparently subjected to similar conditions of darkness, a variation stretching from almost normal eyes to their complete degeneration and the fusion of the eye-stalks with the carapace; and this variation is very difficult to account for. A very frequent condition for crabs living at about 100 fathoms, and even more, is for either the irido-pigment or the retino-pigment to be absent, for the number of ommatidia to be reduced, and for the corneal lenses to be greatly arched. There can be little doubt that these crabs use their eyes, not for mosaic vision, but to obtain the superposition-image characteristic of the Vertebrate eye. In deeper waters, where no daylight penetrates at all, this type of eye is also met with, and also further stages in degeneration where all pigment is absent, and the ommatidia show further signs of reduction and degeneration, e.g. Cyclodorippe dromioides. In a few forms, e.g. Cymonomus granulatus among Brachyura, and numerous Macrura, the ommatidia may entirely disappear, and the eye-stalks may become fused with the carapace or converted into tactile organs.
Progressive stages in degeneration, correlated with the depth in which the animals are found, are afforded by closely related species, or even by individuals of apparently the same species. Thus in the large Serolidae of Antarctic seas, Serolis schytei occurs in 7–128 metres, and has well-developed eyes; S. bronleyana, from 730 to 3600 metres, has small and semi-degenerate eyes; while S. antarctica in 730–2920 metres is completely blind. Lispognathus thompsoni is a deep-water spider-crab, and the individuals taken at various depths are said to exhibit progressive stages in degeneration according to the depth from which they come.
At the same time many anomalies occur which are difficult to explain. In the middle depths, i.e. at about 100 fathoms, side by side with species which have semi-degenerate or, at any rate, poorly pigmented eyes, occur species with intensely pigmented eyes composed of very numerous ommatidia, e.g. the Galatheid Munidopsis and several shrimps, while in the true abysses many of the species have quite normal pigmented eyes. This is especially the case with the deep-sea Pagurids, of which Alcock describes only one species, Parapylocheles scorpio, as having poorly pigmented eyes. An attempt to account for this was made by Milne Edwards and Bouvier,[[119]] who pointed out that the truly deep-sea forms with well-developed eyes were always Crustacea of a roving habit, which were perhaps capable of penetrating into better lit regions, and to whom well-developed eyes might be useful, while the degenerate forms were sluggish. This explanation cannot be held to account for the phenomenon, as too many deep-sea forms with fairly normal eyes are known which are never taken outside deep waters. Doflein (loc. cit.) points out that in the Brachyura of the deep sea there is a remarkable correlation between the degree of degeneration of the eye and the size of the eggs—the large-egged forms having unpigmented and degenerate eyes, while the species with small eggs have pigmented eyes. He supposes that the species with large eggs undergo a direct development without pelagic free-swimming larvae, and that since they never reach the surface their eyes never meet with the necessary stimulus of light for the development of pigment; whereas the small-egged species undergo a pelagic larval existence when this stimulus is present and gives the necessary initiative for the development of the pigment.
Another factor enters into the question of eye-degeneration in the Crustacea. The great majority of deep-sea animals, including many deep-sea Crustacea, are phosphorescent, and it is certain that although daylight never penetrates into the abysses of the ocean, yet there is considerable illumination derived from the phosphorescence of the inhabitants of these regions.