Alcock[[120]] points out in this connexion that the Pagurids, which are conspicuous in the great depths as animals with normally developed eyes, carry about anemones with them, and these organisms are very frequently phosphorescent to a high degree. It may well be, therefore, that the Pagurids are enabled to use their eyes in the normal manner owing to the phosphorescent light which they carry about with them, and this use of phosphorescent light may apply to a number of deep-sea Crustacea whose eyes are not at all or only partially degenerate.
An extremely interesting case of the use of phosphorescent light is given by Chun.[[121]] In a number of Euphausiids occurring in deep waters each compound eye is divided into two parts—a frontal and ventro-lateral—which differ from one another very greatly in the nature and disposition of their ommatidia.
Fig. [104].—Section of eye of Stylocheiron mastigophorum. A, Frontal portion; B, ventro-lateral portion; C, phosphorescent organ; D, entrance of optic nerve; c, corneal lens; cr, crystalline cone; pg, pigment; ret, retinula; rh, rhabdom. (After Chun.)
In the frontal portion (Fig. [104], A) the ommatidia are few in number and long, the corneal lenses are highly arched, and the pigment is reduced to a few clumps in the iris. This part of the eye is evidently adapted for forming a vague superposition-image in the dusk. The ventro-lateral part (B), on the other hand, is composed of numerous small ommatidia, the crystalline cones of which can be completely isolated from one another by the irido-pigment. Immediately below this part of the eye is a phosphorescent organ (C) provided with a lens and tapetum. Chun suggests that the ventro-lateral part of the eye is used for obtaining a clear mosaic image of objects illuminated by the phosphorescent organ, while the frontal part of the eye is used for obtaining general visual impressions in dimly lit regions. This curious differentiation of the eye into two parts apparently only occurs in predaceous animals, which capture their prey alive upon the bottom, and to whom a clear vision of moving organisms is a necessity.
Another instance of Crustaceans making use of their own light is given by Alcock,[[122]] who found two deep-sea prawns, Heterocarpus alphonsi and Aristaeus coruscans, at about 500 fathoms in the Indian Ocean. These animals produce a highly phosphorescent substance which they eject from the antennary glands, and they possess very large, deeply-pigmented eyes.
The whole subject of the modification of the pigment and structure of Crustacean eyes is an interesting one, because it presents us with one of those cases in which the direct response to a stimulus acting within the lifetime of the individual seems to run parallel to the fixed adaptations of a whole species, which have become hereditary and apparently independent of the external stimulus of light or of the absence of light. As far as is known, however, the direct response of the individual to the absence of light is limited to the reduction or disappearance of the pigment, and does not extend to those structural changes in the ommatidia which are characteristic of so many deep-sea forms.
Order II. Decapoda.[[123]]
The Decapoda, together with the Euphausiidae, make up the Division Eucarida, the members of which differ from the Orders hitherto described in a number of characters, e.g. the presence of a carapace covering the whole of the thorax, the absence of a brood-pouch formed of oostegites, the presence of a short heart, of spermatozoa with radiating pseudopodia, and of a complicated larval metamorphosis, of which the Zoaea stages are most prominent.
The Decapoda differ from the Euphausiidae chiefly in the anterior three thoracic limbs being turned forwards towards the mouth to act as maxillipedes, and in the five succeeding thoracic limbs being nearly always uniramous and ambulatory or chelate; there are typically present three serial rows of gills attached to the thoracic segments, an upper series (“pleurobranchiae”) attached to the body-wall above the articulation of the limbs, a middle series (“arthrobranchiae”) attached at the articulation of the limbs, and a lower series (“podobranchiae”) attached to the basal joints of the limbs. These gills are enclosed in a special branchial chamber on each side of the thorax, formed by lateral wings of the carapace known as “branchiostegites.” The gills of each series are never all present in the same animal, the anterior and posterior members showing a special tendency to be reduced and to disappear. In this manner “branchial formulae” can be constructed for the various kinds of Decapods, which differ from the ideal formula in a manner distinctive of each kind. The second maxilla is always provided with an oar-like appendage on its outer margin (exopodite), known as the “scaphognathite,” which, by its rhythmical movement, keeps up a constant current of water through the gill-chamber.