$ Q77. THE CRUSTACEA. 827. 
number of round, pyriform, or cuneiform lenses, the pointed posterior 
extremity of which is surrounded by a pigment matter of usually a deep 
brown or black color, while the rounded anterior extremity is always 
widely protuberant. The optic nerve, before reaching this pigment, divides 
into as many branches as there are lenses. 
With Argulus,° Cyamus,®? and with the Amphipoda,™ there are always 
two considerably flattened eyes ; while with Daphnia, Lynceus, Polyphemus, 
Evadne,™ (the Lophyropoda) and also with the young bivalve Cirripedia,”” 
there is, on the other hand, only a single ocular bulb, spheroidal, and the result 
of the fusion of two eyes; it receives, therefore, the two optic nerves which are 
separated from each other by the median line of the body. With Limnadia, 
and Artemia, of the Phyllopoda, this fusion is less complete, for, upon close 
examination, the line of separation may be seen.°? With many Daphnioidae, 
this cyclopean eye has several muscles, corresponding to the recti muscles of 
the Vertebrata, which give the eyea movement of rotation about its centre. 
With some Crustacea belonging to the orders Amphipoda, Phyllopoda, 
and Poecilopoda, the compound eyes are so‘ modified, that, beneath the 
cornea which is simple, there is another cornea that is faceted. Hach of 
these facets consists of a depression, in which fits the truncated extremity 
of an oblong, conical lens; and the opposite extremity of this lens is sur- 
rounded by pigments, and connected with a filament of the optic nerve. 
A second modification of these compound eyes is also observed with some 
Amphipoda and Phyllopoda. Here, the cornea is likewise double, but be- 
tween the faceted one and the conical lenses, are interposed peculiar lenses 
of an oval form.@? 
The third form of eyes observed with Crustacea has received the name 
of Compound Faceted Hyes. These are found in the genus Scutigera, 
and in the higher groups of Crustdcea, namely: the Stomapoda, and 
Decapoda, with which the eyes are situated at the extremity of two peduncles, 
or, what is more rare, at a point below their extremity.° These peduncies 
are movably inserted on the anterior border of the cephalothorax, and are 
. 
16 Jurine, loc. cit. p. 446, Pl. XXVI. fig. 13, loc. cit. p. 77, Pl. IX. fig. 3,4; and Burmeister, 
and Muller, in Tiedemann’s Zeitsch. f. Physiol. Beitr. p. 17, ‘Taf. I. tig. 3-5. 
IV. p. 97, Taf. VI. fig. 5, 6 (Argulus folia- 21 See Brongniart, joc. cit. p. 85, Pl. XTII. fig. 
ceus).* 38, 4 (Limnadia), and Joly loc. cit. p. 809, Pl. VIL. 
17 Roussel de Vauzeme, loc. cit. p. 242, Pl. fig. 3, Pl. VILL. tig. 24, 26 (saura). In this last- 
VIII. fig. 5. mentioned Crustacean the eyes contain ovoid lenses, 
'3 Muller, in Meckel’s Arch. loc. cit. p. 57, Taf. 22 Daphnia and Evadne ; see Jurine and Lo- 
III. fig. 16, 17 (Gammarus). ven, loc. cit. 
19 The lenses are pyriform with Daphnia (Straus, 23 This modification is found with Amphithoe, 
loc, cit. p. 397, Pl. XXIX. fig. 6,7), cuneiform with Apus, and Limulus; see Milne Edwards, list. 
Polyphemus and Evadne (Jurine, Hist. d. Mon- d. Crust. I. p. 116; Zaddach, loc. cit. p. 45, Tab. 
ocles, Pl. XV. fig. 1-8, and Loven, loc. cit. p. 148, II. fig. 18-24, and Van der Hoeven, loc. cit. p. 23, 
Pi. V.). Taf. IIL. fig. 6, A. B. 
20 It is very remarkable that the Cirripedia, af- 24 Hyperia (Milne Edwards. Jlist. d. Crust. 
ter the disappearance of the simple eye, which, III. p. 74, and Ann.d. Sc. Nat. XX. 1880, p. 388, 
during the embryonic state, is situated on the front, and Mudler, in his" Arch. 1836, p. 102), and 
acquire another, compound but equally transitory. Branchipus (Burmeister, in Muller’s Arch, 
This last is situated at the lower border of the ce- 1835, p. 529, Taf. XIII. fig. 1-4). 
phalic extremity, directly in front of the mouth, dur- The lenses of this last-mentioned Phyllopod are 
ing the period when these animals arecontained situated in the cap-like cavities of the cones, so that 
between two shells, and swim like a Cyris. It this kind of eyes which, moreover, are pedunculated, 
is pedunculated, and has the same structure as form the transition to the faceted ones.* 
that of Daphnia; see Thompson, Zool. Research. 25 With some species of Ocypoda. 
* [ § 277, note16.] For the intimate structure * [ § 277, note 24.] The peculiarity in the struc- 
with many details, of the eyes of Argulus, see Ley- ture of the eye of Branchipus, as above mention- 
dig (loc. cit. Siebold and Koélliker’s Zeitsch. II. ed by Burteister, Leydig (loc. cit. Siebold and 
p. 331, Taf. XX. fig. 1); they are not immovable Kélliker’s Zeitsch. III. p. 295), was unable to 
as Jurine has described. — Ep. verify with Branchipus stagnalis — Ep. 
