326 



THE CRUSTACEA. 



§217. 



remains as a visual organ during the whole life ■,^'^ while, with other Lophy- 

 ropoda, and with the Phyllopoda, it either entirely disappears, "■> or remains 

 in a condition apparently rudimentary, by the side of the other eyes, which 

 are subsequently formed.'"' With certain Ergasilina,<''> and some Lophy- 

 ropoda,<"" with the Caligina,'"' and the males of some parasitic Isopoda,'^> 

 there arc two permanent eyes, right and left, on the vertex of the head. 

 The Poecilopoda, also, have, beside their compound eyes, two simple ones, 

 contiguous on the middle of the forehead.^'''* These simple eyes are also some- 

 times the more numerous, and are then situated on each side of the head, 

 in fours, sixes or eights, in a single or double row, constituting the Omli 

 seriati, as is observed with some Myriapoda; <'^' or they are collected in a 

 thick group of twenty to forty, constituting the Oculi gregaii, as is the case 

 with other Myriapoda, and with the Isopoda.<'^' Each of these eyes has a 

 separate branch of the optic nerve ; this nerve, therefore, divides as many 

 times as there are eyes. 



Another form of eyes which is pretty common among Crustacea, but 

 which has many modifications, has received the name of Compound Un- 



faceted Eyes. 



These organs are composed of a common cornea, covering numerous 

 simple eyes, closely set against each other. They are found in their sim- 

 plest form, with the Cirripedia at a certain epoch of development, with 

 tlie Arguliua, the Laemodipoda, and certain Lophyropoda, Phyllopoda 

 and Amphipoda. Here, directly under the cornea, are a greater or less 



B Lamproglena, Ergasilus {Nordmann, loc. 

 cit. Tab. II. fig. 1, 7), Cyclops, Cyclopsina, Cy- 

 pris, &c. 



7 With Limnadia, and Isaura, it is replaced by 

 a compound eye ; seejoly, Ann. d. Sc. Nat. XVII. 

 PI. IX. fig. 39-41. 



8 This is so with tlie adult individuals of Apus 

 and the Branchiopoda, where the simple embryonic 

 eye persists in an atrophied condition between the 

 two faceted eyes ; see Sckdff'er, Der krebsartige 

 Kiefenfuss, Taf. II. fig. 1 c, 'and Taf. V. fig. 3-5 ; 

 also, Zmldach, loc. cit. p. 48, Tal). II. fig. 18-22, C. 

 and Tab. IV. (Apus); Prevost, in Jurine's Hist, 

 d. Monocles, PI. XX.-X.XI. (Cliirocephatus), and 

 Joly, loc. cit. XUI. PI. VII. (Artcmia). The 

 black spot observed front of the compound eye, 

 with Lynceiis, and certain species of Daphnia, is 

 certainly only the remains of the simple eye ; see 

 Muller, Entomostr. Tab. IX.-XI., and Jurine, 

 Hist. d. Monocles, PI. XV.-XVI. 



But, with this simple rudimentary eye, should 

 not be confounded the problematical vesiculiform 

 organ whicli is found l)ehind the compound eyes of 

 certain Phyllopoda and Lophyropoda. With Apus, 

 this organ contains a nucleus, divided into four 

 parts (Schaffer, loc. cit. Taf. II. fig. 1, b., or Zad- 

 dach, loc. cit. p. 48, Taf. II. fig. 10. P., 25). 

 The vesiculiform body which, with Limnadia., 

 stretches from the inner surface of the head, be- 

 hind the eye, towards the foreliead {Brongniart, 

 loc. cit. p. "83, PI. XIII. fig. 6), may serve, accord- 

 ing to Straus, to fix the animal to foreign bodies ; 

 (see Mus. Senckenb. II. p. 126, or tVrussac, Bull. 



* [ § 277, note 8.] With Artemia and Rranchi- 

 pus, Leydig (loc. cit. Siebold and KoUiker''s 

 Zeitsch. III. p. 295) has found very highly-devel- 

 oped eyes. In structure they correspond to the 

 compound faceted eyes described below. In re- 

 gard to the pigment-spots found on the head of 

 these animals, and regarded as of a visual charac- 

 ter by JoLy and others, this observer considers 



d. Sc. Nat. XXII. 1830, p. 333). With Evadne, 

 there is found at the same place, behind the large 

 eye. a circular muscle, which also, perhaps, is for 

 the attachment of the animal.* 



9 Nicothoe {Rathk6, Nov. Act. Nat. Cut. XX. p. 

 102, Tab. V. fig. 1,8.10). 



10 Hersilia, Pe/tidium, &c. (Philippi, in fVieg- 

 mann's Arch. 1839, I. p. 128, Taf. IV. fig. 9, 13, 

 or Mi/ne Edwards, Hist. d. Crust. PI. XX.XVII.). 



11 Pandarus, Caligus, Trebius, Dinematura^ 

 ha. (Mi/ne Edwards, Hist. d. Crust. PI. 

 XXXVIII., and Krtiyer, in the Naturhist. Tidskr. 

 I., or in the Isis, 1841, p. 188, Taf. I.). 



1::! Phryxus and Bopyrus (Rathki, Nov. Act. 

 Nat. Cur. XX. p. 44, Tab. I. fig. 13, in Tab. II. fig. 

 3, and, De Bopyro et Nereide, Tab. I. fig. 2). 



13 See Fan der Hoeven, Recherches, Aic, 23, 

 PI. III. fig. 5, a. a., 6, C. 



14 With Platyulus, there are, on each side, six 

 eyes, arranged in two rows. Scolopendra has 

 four, while with G/omeris, there are eight, which 

 form a simple arcuate row on each side ; see Mtil- 

 ler, in Meckel's Arch. 1829, p. 40, Tab. III. fig. 

 3, 4, also Kutorga, loc. cit. p. 17, Tab. III. fig. 3, 

 4 (Scolopendra), and Brandt, Mediz. Zool. II. 

 p. 99, Taf. XV. fig. 43 (Glomeris). 



15 Treviranus, Vermisch. Schrift. II. p. 32, 

 Taf. VII. fig. 1 (Lithobius) and Muller, in Meck- 

 el's Ai-ch. 1829, p. 43 (Jiilus) ; see also Trevi- 

 ranus loc. cit. I. p. 64, Taf. I.X. fig. 54 (Porcellio), 

 Muller, loc. cit. p. 42, Taf. III. fig. 5, 6 (Cyino- 

 thoa), and Lereboullet, Ann. d. Sc. Nat. loc. cit. 

 p. 107, PI. IV. fig. 2, 2''- (Lygidium). 



them as mere accumulations of pigment granules, 

 having no special function whatever. This natu- 

 ralist alludes, also, to the problematical body above 

 mentioned. He did not observe it with Artemia, 

 but it was present with Branchipus, and larger In 

 the larval than in the adult conditions. He hesi- 

 tates to express an opinion as to its nature. — Ed. 



