<§« 277. THE CRUSTACEA. 327 



nunibei" of round, pjriform, or cuneiform lense?, 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,'-^^'''' Cyaivus,^^^^ and with the Amphipoda,*^^' there are always 

 two consideralily flattened eyes ; while with Dajj/inia, Lyncens, Polyphemiis, 

 Eijadne,^^''^'' (the Lophyropoda) and also with the young bivalve Cirri|)edia,'-'" 

 there is, on the other hand, only a single ocular bulb, spheroidal, and the result 

 of the fusion of two -js ; it receives, therefore, the two optic nerves which are 

 separated from each oii-or 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 Daphnioi'dae, 

 this Cyclopean eye hns several muscles,- corresponding to the recti muscles of 

 the Vertebrata, which give the eye a movement oF rotation about its centre.'"^ 



With some Crustacea bolüiigiug to the orders Amphipoda, Phyllopoda, 

 and Poecilopoda, the oo.upound 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 Jurfne, loc. cit. p. 446, PI. XXVI. fig. 1.3, loc. cit. p. 77, PI. IX. fi?. 3, 4 ; and Burmeister, 



and Müller, in Tiedemann^s Zeitsch. f. Pliysiul. Beitr. p. 17, Taf. I. fig-, 'd-ä. 



IV. p. 97, Taf. VI. fig. 5, 6 {Ar^ulus folia- ^1 See Brnnsniart, \»c. cii. \>. 85, PI. XIII. fig. 



ctus)." 3, 4 I^Limnadin), and Joty loc. cit. p. Ö09, PI. VII. 



iT Roussel de Vauzime, loo. cit. p. 242, PI. fig. 3, PI. VIII. tig. 24, 'lü {Isaura). In this last- 



VIII. fig. 5. mentioned Crustacean the eyes contain ovoid lenses. 



IS Müller, in MeckePs Arch. loc. cit. p. 57, Taf. ^-' Daphnia and Evadne ; see Jurine and Lo- 



III. fig. 16, 17 (Lianunarus). ven, l>c. cit. 



I'J The lenses are pyrir)rm with DapAnia (S<rn!w.5, ^a This modification is found with AmpliithoSy 



loc. cit. p. 397, PI. XXIX. fig. 6, 7), cuneiform with Apun, -aqA Limulus : see Milne Edwards, llist,. 



Polyphevius and Evadne (^Jurine, Hist. d. Mon- d. Crust. I. p. 116 ; Zaddacli, loc. cit. p. 45, Tab. 



ocles, PI. XV. fig. 1-3, and Loven, loc. cit. p. 14S, II. fig. 18-24, and Van der Hoeven, loc. cit. p. 33, 



PI. v.). Taf. III. fig. 6, A. B. 



20 It is very remarkable that the Cirripedia, af- ^4 Hyperia {Mihie Edwards. Hist. d. Crust, 



ter the disappearance of the simple eye, which, III. p. 74, and Ann. d. Sc. Nat. XX. 1830, p. 388, 



during the embryonic state, is situated on the front, and Müller, in his Arch. 1836, p. 102), and 



acquire another, compound but equally transitory. Branchipus {Burmeister, in MillleT''s Arch. 



This last is situated at the lower border of the ce- 1835, p. 529, Taf. XIII. fig. 1-4). 



phalicextremity, directly in front of the mouth, dur- The lenses of this last-mentioned Phyllopod are 



ing the period when these animals are c^ntaiaed situated in the cap-like cavities of the cones, so that 



between two shells, and swim like ■a.Cj.iris. 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. ^ AVith some species of Ocypoda. 



* \ § 277, note 16.] For the intimate structure * [ § 277, note 24.] The peculiarity in the stnic- 

 with many details, of the eyes of Ar^ulus, see Ley- ture of the eye of Branchipus, as above mention- 

 dig- (loc. cit. Siebold and Kölliker''s Zeitsch. II. ed by Burmeister, Leydig (loc. cit. Siebold and 

 p. 331, Taf. XX. fig. 1) -, they are not immovable Kölliker''s Zeitsch. III. p. 296), was unable to 

 as Jurine has described. — Ed. verify with Branchipus stagnalis — Ed. 



