362 



THE CRUSTACEA. 



§294. 



veloped the ventral portion together with the abdominal cord of the future 

 embryo ; while, at the opposite pole, where the borders of the disc meet, the 

 dorsal portion of the animal appears. Quite early, the blastoderma can be 

 seen composed of an external or serous, and of an internal or mucous 

 layer. This last, after having enveloped the entire vitellus, is changed 

 gradually into the alimentary canal. The hepatic organs are only dever- 

 ticuli of this last, while the antennae, the oral apparatus, the feet, and the 

 branchiae, are developed from the serous layer. 



The embryos, thus formed, differ considerably, and their form is often so 

 dissimilar from that of the adult animal, that, during their ulterior develop- 

 ment, there is a real metamorphosis, which takes place by more or less 

 numerous stages coincident with the act of moulting. 



An embryonic type quite general among the lower Crustacea, that is, 

 the Cirripedia, Siphonostoma, Lophyropoda and Phyllopoda, is that which 

 was first observed with Cyclops. There is here a long series of metamor- 

 phoses. The monocle-like larvae have an ovoid, unarticulated body, usually 

 provided with a single, simple eye, and two or three pairs of oar-like ap- 

 pendages covered with long hairs. '^' 



With some Brachyura, there is an equally well-marked metamorphosis ; 

 for, in leaving the egg, they have a long tail and two very large eyes ; but 

 with the first moulting they acquire two enormous, spur-like apophyses, one 

 on the front, and the other on the back.^^' 



3 It is remarkalile that the young Cirripedia, 

 which are hexapod, have the characteristics of 

 the larvae of Monocles ; see Thompson, Zool. 

 Research, loc. cit. p. 69, PI. IX. iBalanus) ; Bur- 

 meister, Beitr. loc. cit. p. 12, Taf. I. {Lepas) ; 

 Goodsir, Edinb. New Philos. Jour. No. 69, July, 

 1813, p. 97, PI. III. IV., or Isis, 184-t, p. 901, 

 Taf. I. fig. 8, 11-17 (Balanus). Tlie larvae of 

 these Crustacea, before becoming fixed in order 

 to undergo their metamorphoses, change into a 

 bivalve animal resembling Cypris. Among tlie 

 Siphonostoma, the monocle-like embryos are very 

 general. Nordmann (loc. cit. p. 11, &c., Taf. II.- 

 VII.) has recognized larvae of tliis kind, some v/ith 

 three {Ergasilus and Lernaeocera),a.nA others witli 

 only two {Achtheres and Tracheliastes) pairs of 

 feet. According to Kollar (loc. cit. p. 87, Taf. X. 

 fig. 10), the embryos of Basanistes are monocle- 

 like and have six feet, as are also those of 

 Lernaeopoda described by Rathke (Zur Mor- 

 phol. &c. p. 34, Taf. I.). Goodsir (loc. cit. No. 

 65, July, 1842, p. 178, PI. III. fig. 19-23) has 

 observed embryos with four feet in the eggs of 

 Callous. The larvae of NicotJioe {Ratkia, Nov. 

 Act. Nat. Cur. X.X. p. 109, Tab. V. fig. 8-10) and 

 of Argulus (Mu/ler, Entomostr. p. 122, Tab. X.X. 

 fig. 2, and Jurine, loc. cit. p. 453, PI. XXVI. fig. 

 4) form an exception in this respect, for when 

 they leave the egg they have two simple eyes, an 



articulated body, and more than three pairs of feet. 

 Those of the Cyclopidae which have six feet have 

 been known a long time. But Mailer (Entomustr. 

 p. 39, Taf. I. II.) formerly divided them under the 

 names of Nauplius a.nd Amymone. See Degeer 

 Abhandl. &c. VII. p. 181, Taf. XXX. {Cyclops) ; 

 Ramdohr, loc. cit. p. 5, &c., Taf I. III. ; but 

 especially Jurine, Hist. d. Monocl. p. 15, &c., PI. 

 I.-VII. {Cyclops and Cyclopsinn). The young 

 Daphnio'idae and Cyprido'idae, on tlie contrary, re- 

 semble the adults on their escape from the egg. The 

 simple eye is evidently the result of a very early 

 fusion of two eyes ; see Jurine, loc. cit. p. 113, 

 PI. VIII. IX. {Daphnia and Cypris) ; Ralhki, 

 Abhandl. z. Bildungs u. Entwickelungsgesch. &c. 

 p. 85 {Daphnia and Lijnceu.s) ; Baird, Magaz. 

 of Zool. and Bot. I. p. 522, and II. PI. V. fig. 

 12 {Cypris) ; finally Lov^n, loc. cit. p. 161, Taf. 

 V. fig. 12 {Evadne). Of the Phyllopoda, the mo- 

 nocle embryos of the Apodidae have two pairs 

 of feet, while those of the Branchiopoda have three ; 

 see Schaeff'er, l)er krebsartige Kiefenf. p. 118, Taf. 

 I. fig. 3 ; and Zaddach, loc. cit. \>. 55, Tab. IV. 

 fig. 1-3 {Apus) ; Joly, loc. cit. p. 321, PI. IX. fig. 

 39 {Isaura) ; Prevost, in Jurine''s Hist. d. Mon- 

 ocl. p. 214, PI. XX. fig. 9 {Chirocephalus); and 

 Joly, loc. cit. p. 257, PI. VII. fig. 4 {Artemia)* 



4 These embryos with such singular forms, have, 

 hitherto, been figured as separate genera under the 



* [ § 294, note 3.] For many highly-interesting 

 details on the economy of the Entomostraca, see 

 Baird (British Entomostr. &c. loc. cit. passim). 

 These details with their corres])onding figures will 

 render clear many obscure economical points al- 

 luded to above. For the embryology of Argulus, 

 Artemia and Branchipus, see Ley dig, loc. cit. 

 Siebold and Kolliker''s Zeitschrift, II. p. 344, and 

 III. p. 304. The descriptions of this observer are 

 quite rich in details upon the successive appear- 



ances of the different organs. Arguliis is quite 

 well developed when hatched, its muscles are trans- 

 versely striated and the locomotive organs well 

 formed. Artemia has, at this jieriod, two anten- 

 nae, two pairs of feet on the head, and the red pig- 

 ment spots on the forehead, but these last have as 

 yet no light-refracting body. The muscles are still 

 without striae, and even here and there are filled 

 with vitelline globules. The heart and biood-cir- 

 culation are still unformed. — Ed. 



