862 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. 
8 It is remarkable that the young Cirripedia, 
which are hexapod, have the characteristics of 
the larvae of Monocles; see Thompson, Zool. 
Research. loc. cit. p. 69, Pl. LX. (Balanus) ; Bur- 
meister, Beitr. loc. cit. p. 12, Taf. I. (Lepas) ; 
Goodsir, Edinb. New Philos. Jour. No. 69, July, 
1848, p. 97, Pl. IU. IV., or Isis, -1844, p. 901, 
Taf. I. fig. 8, 11-17 (Balanus). The larvae of 
these Crustacea, before becoming fixed in order 
to undergo their metamorphoses, change into a 
bivalve animal resembling Cypris. Among the 
Siphonostoma, the monocle-like embryos are very 
general. Nordmann (loc. cit. p.11, &c., Taf. IL— 
VII.) has recognized larvae of this kind, some with 
three (Ergasilus and Lernaeocera),and others with 
only two (Achtheres and Tracheliastes) pairs of 
feet. According to Koldar (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 Rathké (Zur Mor- 
phol. &c. p. 34, Taf. I.). Goodsir (lov. cit. No. 
65, July, 1842, p. 178, Pl. III. fig. 19-23) has 
observed embryos with four feet in the eggs of 
Caligus. The larvae of Nicothoé (Rathké, Nov. 
Act. Nat. Cur. XX. p. 109, Tab. V. fig. 8-10) and 
of Argulus (Muller, Entomostr. p. 122, Tab. XX. 
fig. 2, and Jurine, loc. cit. p. 453, Pl. XXVI. fig. 
4) form an exception in this respect, for when 
-they leave the egg they have two simple eyes, an 
* [ § 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 corresponding figures will 
render clear many obscure economical points al- 
luded to above. For the embryology of Argulus, 
Artemia and Branchipus, see Leydig, loc. cit. 
Siebold and Kélliker’s Zeitschrift, II. p. 344, and 
TII. p. 304. The descriptions of this observer are 
quite rich in details upon the successive appear- 
articulated body, and more than three pairs of feet. 
Those of the Cyclopidae which have six feet have 
been known a long time. But Miller (Entomostr. 
p. 39, Taf. I. II.) formerly divided them under the 
names of Naupliusand Amymone. See Degeer 
Abhandl. &c. VIL. p. 181, Taf. XXX. (Cyclops) ; 
Ramdohr, loc. cit. p. 5, &c., Taf. I. IT. 3; but 
especially Jurine, Hist. d. Monocl. p. 15, &c., Pl. 
I.-VII. (Cyclops and Cyclopsina). The young 
Daphnioidae and Cypridofdae, on the contrary, re- 
semble the adults on their escape from theegg. The 
simple eye is evidently the result of a very early 
fusion of two eyes; see Jurine, loc. cit. p. 113, 
Pl. VILL. IX. (Daphnia and Cypris); Rathké, 
Abhandl. z. Bildungs u. Entwickelungsgesch. &c. 
p. 85 (Daphnia and Lynceus); Baird, Magaz. 
of Zool. and Bot. I. p. 522, and II. Pl. V. fig. 
12 {Cupris) j finally Lovén, 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 Schaeffer, Der krebsartige Kiefenf. p. 118, Taf. 
I. fig. 35 and Zaddach, loc. cit. p. 55, Tab. IV. 
fig. 1-8 (Apus) ; Joly, loc. cit. p. 321, Pl. IX. fig. 
39 (saura) ; Prevost, in Jurine’s Hist. d. Mon- 
odl. p. 214, Pl. XX. fig. 9 (Chirocephalus) ; and 
Joly, loc. cit. p. 257, Pl. VII. fig. 4 (Artemia).* 
4 These embryos with such singular forms, have, 
hitherto, been figured as separate genera under the 
ances of the different organs. Argulus is quite 
well developed when hatched, its muscles are trans- 
versely striated and the locomotive organs well 
formed. Artemia has, at this period, 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. — Ep. 
