§ 277. 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 Ai'gulus,'-^'^^ Cyamus,^^^'' and with the Amphipoda,''**' there are always 

 two considerably flattened ej'es ; 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, theretbre, the two optic nerves which are 

 separated from each other by the median line of the body. With Limnddia, 

 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 Amphipoda, 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 

 Amjjhipoda 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 



16 Jurine, Inc. cit. p. 446, PI. XXVI. fiff. 13, loc. cit. p. 77, PI. IX. fifr. 3, 4 ; and Burmeister, 

 and Mxiller, ia Tiedemann's Zeitsch. f. Physiol. Beitr. p. 17, Taf. I. fig. 3-5. 



IV. p. 97, Taf. VX. fig. 5, 6 {Arg:alus folia- 21 See Bronffnjart, loc. cit. p. 85, PI. XIII. fig. 

 ctus).' 3, 4 {Limnadin), and Joly loc. cit. p. 309, PI. VII. 



17 Roussel de Vauzime, loc. cit. p. 242, PI. fig. 3, PI. VIII. tig. 24, 26 (Isaura). In this hist- 

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



'8 Mailer, in MeckeVs Arch. loc. cit. p. 57, Taf. -'a Daphnia and Evadne ; see Jurine and Lo- 

 ll^, fig. 16, 17 {Gammanix). ven, loc. cit. 



IS* Tlie lenses are pyvifurm with jDa;)An2« (S/r«i«, ^3 This modification is found with Awphithoe, 



loc. cit. p. 397, PI. XXIX. fig. 6, 7), cuneiform with Apns,ai\A Limxlus ; see Milne Edwards, Hist. 



Polyphemus and Evadne {Jurine, Hist. d. Slon- d. Crust. I. p. 116 ; Zaddacli, loc. cii. p. 45, Tab. 



ocles, PI. XV. fig. 1-3, and Lovin, loc. cit. p. 148, II. fig. 18-24, and Fan rfer/Zoeucn, loc. cit. p. 23, 



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



20 It is very remarkable that the Cirripedin, af- -'< Hijperia {Milne Edwards. Hist. d. Crust, 



ter the disappearance of the simple eye, which, III. p. 74, and Ann. d. Sc. Nat. XX. 1830, p. 383, 



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



acquire another, compound but equally transitory. Rrnnchipus {liurmeister, in M-Uller^s Arch. 



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



phalic e.\tremity, directly in front of the mouth, dur- The lenses of this last-mentioned Phyllopod are 



lug the period when these animals are contained situated in the cap-like cavities of the cones, so that 



between two shells, and swim like aC'jpris. It this kind of eyes which, moreover, are pedunculated, 



is pedunculated, and has the same structure as form the transition to the faceted ones.* 



tha.to{ Daphnia ) see Thompson, Zool. Research. /!0 With some species of Oci/;;oda. 



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



with many details, oftheeyes of Ar^ulus, seeLey- ture of the eye of Branchipus, as above mention- 



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



p. 331, Taf. XX . fig. 1) ; they are not mimovable Kolliker's Zeitsch. III. p. 295), was unable to 



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



