S^ 277. 



THE CRUSTACEA. 



327 



number of round, pjriform, 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 Argidus/^^^ Cyamus,^^'^ and with the Amphipoda,*^* there are always 

 two considerably flattened eyes ; while with Dapknia, 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 eye a movement of rotation about its centre. <^-^ 



With some Crustacea belonging to the orders Aniphipoda, Phyllopoda, 

 and Poecilopoda, the compound eyes are so modified, that, beneath the 

 cornea which is simple, there is another cornea that is faceted. Each 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 Eyes. These are found in the genus Scutigera, 

 and in the higher groups of Crustacea, 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 peduncles 

 are movably inserted on the anterior border of the cephalothorax, and are 



IG Jurine, loc. cit. p. 446, PI. XXVI. fig. 13, 

 and Mailer, in Tiedemann's Zeitsch. f. Physiol. 

 IV. p. 97, Taf. VI. fig. 5, 6 (Ar^ulus foUa- 

 ceus).* 



17 Roussel de f'auzeme, loc. cit. p. 242, PI. 

 VIII. fig. 5. 



l*i MiLller, in MeckePs Arch. loc. cit. p. 57, Taf. 

 m. fig. 16, 17 (Gammarus). 



19 The lenses are pyriform with Daphnia (Straus, 

 loc. cit. p. 397, PI. XXIX. fig. 6, 7), cuneiform with 

 Polyphemus and Evadne {Jtirine, Hist. d. Mon- 

 ocles, PI. XV. fig. 1-3, and Lovin, toe. cit. p. 148, 



PI. v.). 



20 It is very remarkable that the Cirripedia, af- 

 ter the disappearance of the simple eye, which, 

 during the embryonic state, is situated on the front, 

 acquire another, compound but equally transitory. 

 This last is situated at the lower border of the ce- 

 phalic extremity, directly in front of the mouth, dur- 

 ing the period when these animals are contained 

 between two shells, and swim like a C'jpris. It 

 is pedunculated, and has the same structure as 

 that of Daphnia ; see Thompson, Zool. Research. 



* [ §. 277, note 16.] For the intimate structure 

 with many details, oftheeyes of Ar^ulus, seeLey- 

 dig (loc. cit. Siebold and KiiUiker^s Zeitsch. II. 

 p. 331, Taf. XX . fig. 1) ; they are not immovable 

 as Jurine has described. — Ed. 



loc. cit. p. 77, PI. IX. fig. 3, 4 ; and Burmeister, 

 Beitr. p. 17, Taf. I. fig. 3-5. 



21 See Brons:niart, loc. cit. p. So, PI. XIII. fig. 

 3, 4 {Limnadia), and Joly loc. cit. p. 309, PI. VII. 

 fig. 3, PI. VIII. fig. 24, 26 (Isaura). In this last- 

 mentioned Crustacean tlie eyes contain ovoid lenses. 



22 Daphnia and Evadne ; see Jurine and Lo- 

 ven, loc. cit. 



2.3 This modification is found with Amphithoe, 

 Apus, and Limulus ; see Milne Edwards, Hist, 

 d. Crust. I. p. 116 ; Zaddach, loc. cit. p. 45, Tab. 



II. fig. 18-24, and Fan der Hoeven, loc. cit. p. 23, 

 Taf. ill. fig. 6, A. B. 



24 Hyperia (Milne Edwards. Hi.=it. d. Crust. 



III. p. 74, and Ann. d. Sc. Nat. ,XX. 1830, p. 388, 

 and Muller, in his Arch. 1836, p. 102), and 

 Branchipus {Burmeister, in Muller''s Arch. 

 1835, p. 529, Taf. XIII. fig. 1-4). 



The lenses of this last-mentioned Phyllopod are 

 situated in the cap-like cavities of the cones, so that 

 this kind of eyes which, moreover, are pedunculated, 

 form the transition to the faceted ones.* 



25 With some species of Ocypoda. 



* I § 277, note 24.] The peculiarity in the struc- 

 ture of the eye of Branchipus, as above mention- 

 ed by Burmeister, Leydig (loc. cit. Siebold and 

 Kolliker's Zeitsch. III. p. 295), was unable to 

 verify with Branchipus stagnalis — Ed. 



