038 CRUSTACEA indeed, if we except certain parasitic Isopodous forms and t!ie Cirripedia and Rhizocephala, we shall find that the faculty of sight is possessed by the whole class. 1 Even in those exceptional cases in which the eyes are aborted, we find that in the earlier and larval stages of their existence the parasitic and sedentary forms possessed eyes, and it is only as an effect of a kind of retrograde metamorphosis which the animal undergoes that the organs of vision dis appear in the adult. 2 Two forms of visual organs are met with in the Crustacea, namely, smooth or simple eyes (ocelli or stemmata) and compound eyes ; but though there are some few forms in which (as in Apus and Limulus) both ocelli and compound eyes are present, the latter form of eyes is that most generally met with in the class. The structure of the simple eye does not differ greatly from that observed among higher animals. There is, firstly, a transparent cornea, smooth and rounded, which is, in fact, only a part of the general tegumentary covering modified. Immediately behind the cornea is the crystalline lens, generally of a spherical form ; this is again followed by a gelatinous mass analogous to the vitreous humour, and this mass is, in its turn, in contact with the extremity of the optic nerve. A layer of pigment of a deep colour envelops the whole of these parts, lining the internal wall of the globe of the eye up to the point at which the cornea begins to be formed by the thinning of the tegumentary envelope rendering it transparent. The number of these simple eyes does not exceed two or three. In the Branchiopoda (Nebalia, Branchipus, Daphnia) behind a simple cornea, undivided externally, we find a variable number of distinct crystalline lenses and vitreous humours, each included in a pigmentary cell, and ter minating by contact with the optic nerve. These are, no doubt, an aggregation of stemmata under a common cornea. In some of the Edriophthalmia a still further advance to a true compound eye is met with. In these the cornea appears to consist of two transparent laminae, the external layer being smooth and the internal one facetted, each facet being a distinct cornea resting on a separate crystalline lens of its own. In the compound eye, properly so called, the two membranes, external and internal, constituting the cornea, are both divided into facets, each facet seemingly being equivalent to a distinct ocellus, furnished with its own crystalline cone (or lens) and nerve rod ; each invested with its own pigment coating, which, being darker at intervals, gives to a section through the compound eye as a whole the appearance of pigment-bands repeated at various depths beneath the cornea, and in front of the expanded termination of the optic nerve (retina). Although these facets are always hexagonal in the eye of an insect, they are variable in form among the Crustacea. Thus in HomaruSy Astacus, Penceus, Galathea, and Scyllarus, the facets are square ; whilst in Pagurus, Squilla, Gebia, Cal- lianassa, and the crabs, they are hexagonal. In Limulus and the Trilobites the lenses are round, not being in actual close contact with each other. Milne-Edwards mentions that in Idotea each facet has a kind of supplemental lens of a circular shape set within the cornea in front of each 1 The fact that Niphargus, Crangonyx, and some other dwellers in subterranean waters, as well as Callianassa Macandrci, a burrowing marine crustacean, are blind, is certainly the result of their habitats, not a normal state of the organs of vision. In the Mammoth Cave, Kentucky, and the caves of Carniola and Adelsberg, Crustacea, insects, and other animals have been met with, all blind or with but imperfect organs of vision. 2 In the Cirripedia the individuals are hermaphrodite, fixed when adult, and all blind (unless the complemental male of Ibla Cumingii bean exception, see Darwin s Man. Cirripedia, Ray Soc., p. 196), but in the parasitic Isopoda, and in many of the Copepoda, it is the female alone which is so remarkably transformed, whilst the male retains his powers of sight, his freedom, and his normal aspect. cau- datus), U. Silurian. (After Bucklaud). the entire eye ; g, the genal border of head ; 6, a portion of the facet ted surface, much en larged. proper crystalline lens, and equal in size to the corneal facet, and apparently evolved in the substance of the cornea itself, but under favourable circumstances capable of being detached from it. In Phacops caudatus the small circular lenses of the external compound eyes drop out, leaving a corresponding concavity beneath. Emmerich long since proposed to use the external characters of the eyes of Trilobites as a means of classifica tion, dividing them into " hyaline "- eyed and "facetted "-eyed groups; but he does not seem to have been aware of the perfect analogy which the struc ture of the eyes of the modern Edri- ophthalmia afford in illustration of " "*" T "7T" this ancient and extinct group. ^ Stemmata or ocelli are always im- FIG. 11. Eye of Trilo- movable and sessile; the compound kite (Phacops eyes with smooth cornere, although usually sessile, are, however, occasion ally supported on pedicles, as in Nebalia and Eranchipus. The compound facetted eyes are subject to the same variations, genera being found with hyaline and facetted cornese in the same order. In some of the compound sessile eyes the facets are round; but in all the pedunculated compound eyes they are either square or hexagonal. The peduncles supporting the eyes in the Stomapoda and Decapoda vary greatly in length, but every considera tion tends to the conclusion that these movable eye-stalks are really the pair of appendages of the first cephalic ring. Indeed, in Sguilla one is actually able to separate the eye-stalks with the segment upon which they are borne from the cephalic shield. In Macrophthalnms and some other crabs the eye-stalk is of very considerable length (see fig. 65), extending even to the outer angle of the front of the carapace, which is furnished with a long groove or furrow into which the eye can be folded down, and so placed out of reach of injury when not in active use. This furrow is called the orbital fossa. ORGAN OP HEARING. Milne-Edwards, Owen, Bell, and others consider the external organ connected with the sense of hearing to be situated on the first joint of the outer and larger antennae in the lobster and other Macroura, and to consist of a conical process beneath which is a cavity having a round orifice closed by a membrane. Behind the process, and connected with the cavity, is a large sac filled with a clear liquor ; a nerve arising in common with the external antennal nerve is spread upon the delicate walls of the supposed acoustic sac. In Maia and other crabs the membrane is replaced by a movable calcareous disc pierced by a small oval opening, over which is stretched a thin elastic membrane (which might be termed the internal auditory mem brane), near to which the auditory nerve appears to ter- m mate. The auditory apparatus of the Crustacea consequently consists essentially of a cavity full of fluid, over which a ne,.ve adapted to perceive sonorous impulses is distributed, assisted by an elastic membrane, and placed near the base of the antennae which, like a rigid stem, assists in rendering certain vibrations perceptible. In both the lobster and the crab a gland filled with a greenish substance is connected with the membranous sac. This structure and the absence of otolites has led Farre to
suggest that the organ may be olfactory ; but the chief