CRUSTACEA. 
410 
The singular fossils called Trilobites, of which M. Brongniart has furnished an 
excellent monograph, being considered by him and many other naturalists as crus- 
taceous animals allied to the Entomostraca, we have introduced them concisely at the 
end of that section. 
, FIRST GENERAL DIVISION. 
CRUSTACEA MALACOSTRACA,— 
Which are divisible into those which have the eyes placed on a moveable foot-stalk, 
and those which have them sessile and fixed. 
Those Malacostraca with the eyes placed on a moveable foot-stalk, articulated 
[at the base, Podopthalma, Leach], composing the orders Decapoda and Stomapoda, 
have many characters in common. A large shield, sometimes divided into two parts, 
and termed the shell or carapax, covers a large portion of the front of the body. They 
have four antennae, the exterior pair being longest and simple, whilst the intermediate 
pair is shorter, and divided at the tip into two branches in the crabs, and into three in 
many of the Macrura ; two mandibles, each with a three-jointed palpus near the base, 
a bilobed tongue, two pairs of maxillae, three pairs of foot-jaws, the two outer pairs 
being in some [Squilla] transformed into claws, and ten or fourteen (in those species 
which have the four outer foot-jaws leg- shaped) legs. 
In the majority the branchiae, of which there are seven pairs, are hidden beneath the 
lateral margins of the carapax, the two anterior pairs being fixed at the base of the two 
exterior pairs of foot-jaws, and the others at the base of the true legs. In the other 
species [Squilla, &c.] they form brushes attached to the five pairs of sub-abdominal 
swimming legs. The under side of this post-abdomen is likewise furnished in the 
others with four or five pairs of bifid appendages. 
THE FIRST ORDER OF CRUSTACEA. 
decapoda (TEN-FOOTED). 
I 
The head is compactly soldered to the thorax, and covered, as well as that part of the body, 
by a large and continuous shell or carapax, generally exhibiting on its surface various | 
impressed lines, dividing it into regions corresponding with the internal organs, and which 1 1 
have been ingeniously named by M. Desmarest. The circulatory system differs in some 
respects from that of the other Crustacea; the blood before reaching the branchiee to be 
oxygenated passing through two great reservoirs, one on each side, above the legs, analogous! 
to the lateral hearts of the Cephalopods, according to Milne Edwards, Audouin, and Cuvier. 
1. Aspidostraca, divided into five sub-orders. 
1. Parasita, including the Penellina, Lernasoda, Ergasilina, 
Caligina, and Argulina. 
2. Lophyropoda, including the Ostracoda, Cladocera, and 
Cyclopida, 
3. Phyllopoda, including the Gymnota (Branchipus), and As- 
pidophora (Apus). 
4. Cirripedia, including the Lepadeaand Balanoda. 
5. Precilopoda, including only Xiphosura. 
2. Thoracostraca (Podopthalma, Leach), divided into two suborders, 
Decapoda and Stomapoda. 
3. Arthrostraca (Edriopthalrna, Leach), divided into nine minor 
divisions, Gammarina, Typhina, Loemodipoda, Epicarida, Cymo- 
thoadae, Sphoeromatoda, Asellina, Tdotoda, and Oniscoda. 
De Haan, in his magnificent work upon the Crustacea of Japan, 
adopting the quinarian circular system of M’Leay, divides the class 
into five orders, — Decapoda, Stomapoda, Tetradecapoda(EJdr£qptAafma, 
Leach), Lophyropoda, and Phyllopoda. M. Duverney has, within the 
last few months, submitted a Memoir to the Academic des Sciences at 
Paris, proposing a new classification of the Crustacea according to the 
organs of respiration, dividing the class into three principal groups, 
Nudibranchiae, Cryptobranchiae, and Lamellibranchiae ; but the adop- 
tion of this, like any other single character, has had the effect of 
breaking the most natural relations.] 
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