Eyes are absent in some Crustaceans which live subterraneanly. They are confined to the head, but in the Enphausidae (Schizopoda) accessory eyes or luminous organs (?) occur on the coxae of the second thoracic limbs, on the bases of the penultimate gills, and between the four first abdominal limbs. The eye is lost during development in Cirripedia, and many parasitic Copepoda, In some free living families of the last named order, e. g. Cyclopidae, there is an azygos median eye, composed of a ventral and two dorsal pigment plates imbedding a number of refractile cells. This azygos eye, which is found in the Nauplius form of all Crustacea, may persist as a rudiment in some Branchiopoda and those Copepoda which like the Pontellidae and Argulus possess the two laterally-placed compound eyes found in all other Crustacea. These lateral eyes may however fuse in the course of development as in Cladocera, and the Cypridae and Cytheridae among Ostracoda. In the Cladocera they also become inclosed in a sinus by the growth of an integumental fold. The eye is moveable in Cladocera, in the Ostracode Cypridinidae, and the Argulidae; mounted on a fixed stalk in the Branchiopod genus Branchipus, on a moveable stalk in Podophthalmata. The compound eye has the usual Arthropodan structure.
The corneal cuticula is facetted in the Mala-costraca, and the corneal lenses in Isopoda are of great size, but in other Crustacea the cuticula is of even thickness. A distinct layer of hypo-dermis cells intervenes between the vitrellae and corneal cuticula in Isopoda and Amphipoda. The crystalline cones are composed of two segments in Isopoda, Amphipoda, and Schizopoda; of five in Cladocera; of four in other Crustacea; the retinulae are five in number in Estheridae (Branchiopoda), the Cladocera, and Amphipoda, seven in other Crustacea with possibly the exception of the Schizopoda. The Malacostraca possess four distinct ganglionic swellings in the course of the optic nerve. The 'eye-elements' are remarkably distinct from one another in Cladocera, Isopoda, and some Amphipoda. In the Isopoda they are separated by intervening pigmented hypodermis cells. The Copepodan family Cory-caeidae differ in the structure of their eyes from all other Crustacea. A large soft lens lies immediately beneath the corneal cuticula, and the crystalline cone with the ommateum is situate at some considerable distance behind it1. Olfactory setae are generally found on the first antennae.
1 The Phronimidae among Amphipoda possess four eyes evidently formed by the division of the two eyes of other Amphipods. The pair on each side of the head are supplied by branches of the same nerve; and in Gammarus pulex the single eye on each side is constricted laterally. The
The Podophthalmata possess auditory organs, either open sacs provided with auditory hairs and inclosing foreign bodies as otoliths and placed in the basal joint of the first antenna in the Decapoda, or closed sacs containing a laminated otolith and situate in the endopodite of the swimmeret as in many Schizopoda. Auditory hairs are said also to occur on other parts of the body in the groups named.
Mouth and digestive apparatus are wanting in the Rhizocephala among Cirripedia, and the animal is attached to its host by branching processes which spring from its head and penetrate among the viscera. The tract is rudimentary in Proteolepas and many male Cirripedia. In other Crustacea the mouth lies between the mandibles and has an upper lip or epistoma, and a lower lip or metastoma1. The latter is often bilobed, and is known in that case as paragnatha. In Euphausia and perhaps in Malacostraca generally the paragnatha are derived from the basal part of the first maxillae. The stomodaeum or oesophageal section of the tract is well developed and often dilated posteriorly. The dilatation in higher Crustacea-is generally provided with chitinoid and sometimes calcified plates and teeth, differentiations of the cuticular lining. The mesenteron has no cuticle and corresponds with the archenteron. It varies in length and is rarely convoluted, and is furnished with glandular caeca. These caeca may be short and simple, variable in number, or large branched organs as in Decapoda. The proctodaeum is long in the higher Crustacea, short in the Entomostraca, and is lined by cuticle.
The anus is posterior, and may be dorsal (Copepoda) or ventral (Malacostraca).
The coelome is filled to a variable degree by the muscles of the somites and limbs and the viscera. It contains a blood-plasma in which amoeboid corpuscles are suspended. The plasma is sometimes tinged red by a colouring matter, either haemoglobin (some Phyllopoda and Cypris) or tetronerythin. In other instances, e.g. Homarus, Squilla (Decapoda), it contains haemocyanin or haemochromogen. A heart may be absent (some Ostracoda and Copepoda, Cirripedia?). It is placed in the thorax nuclei of the retinulae in Amphipoda are situated in the bases of the cells, which are distinctly separated from their outer parts. The nerves to the refractile cells of the Cyclops eye either unite with their internal pointed ends (Grenacher) or with their outer ends (Hartog).