The eyes of Cirripedes certainly undergo a remarkable series of changes: in the larvæ in the first stage, there is a single eye, perhaps formed by the confluence of two eyes, occupying the normal position in the front of the head: in the second stage, according to Burmeister, the eye has become double, but the two are as yet simple; they are now situated posteriorly to the second pair of antennæ: in the third or pupal stage, they remain in the same situation, but have become compound, of great size, and are attached to the apodemes of the antennæ: in the mature and fourth stage, they have moved someway posteriorly, and again have become simple, of minute size, and are either confluent, as in the Lepadidæ, or tolerably far apart, as in the Balanidæ. It must not be supposed that the eye of the mature Cirripede is metamorphosed from the eye of the pupa, for such is not the case; the new eyes and old eyes being formed someway apart, and frequently both can be seen within the pupa (as in Alcippe, Pl. 23, fig. 16) at the same time. It is scarcely possible that the eye of the larva in the first stage, can be changed into the double eyes of the second stage; though these latter may possibly be multiplied into the eyes of the pupa, as both continue to occupy nearly the same position.[65] [65] Zenker, in his 'Physiological Remarks on the Daphnidæ,' ('Journal of the Microscopical Society,' 1853, p. 274), speaks of a "tripartite azygous eye" as common amongst Crustacea, and as occurring "in conjunction with the aggregated eyes in Artemia, Argulus, &c.; but as appearing regularly in all the Branchiopoda and Siphonostomata as the _earliest_ visual organ." Hence I conclude that this azygous eye is the homologue of that single eye which appears in the earliest larval stage of Cirripedes; and that the compound eyes of the cirripedial pupa, answer to the aggregated eyes of Artemia and Argulus, &c., with the difference, that in these latter genera the single eye is retained. See, also, Von Siebold, 'Anatomie Comparée,' tom. i, p. 435.