534 THE ANIMAL KINGDOM. 



Eyes are absent in some Crustaceans which live subterraneanly. They 

 are confined to the head, but in the Euphausidae (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 it 1 . Olfactory setae are generally found on the first antennae. 



running from one to the next succeeding ganglion of the ventral cord, as described first by Rathke, as 

 well as two recurrent nerves, derived one from each of the two posterior terminal nerves, and 

 supplying the walls of the digestive tract. But he did not find the azygos 'ganglion frontale' 

 of Leydig, placed in front of the supra-oesophageal ganglion to which it is connected ; nor yet the 

 dorsal intestinal nerve of the same observer, or of Lereboullet (see pp. 306-10, op. cit. in note i). 



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 



