410 CRUSTACEA. 



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 Malacostsaca 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). 



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 hnes, dividing it into regions corresponding with the internal organs, and which 

 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 branchia; 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 Peneilina, Lcrnaeodn, Ergasilina, 



Calitfina, and Argulina. 



2. Lophyropoda, including the Ostracoda, Cladocera, and 



Cyclopida. 



3. Pliyllopoda, including the Gymnota (Branchipiis), and As- 



pidophora (Apus). 



4. Cirripedia, including the Lepadeaand Baiauoda. 



5. PflEcilopoda, including only Xiphosura. 

 *2. Thoracostraca [Podupthalma, Leach], divided into two uuborders, 



Decapoda and Stomapoda. 

 3.Atthro.traca(£dno/,M<.;ma, Leach), divided into nine minor "b7;,kY,,g7h; ^ost naturarrelritiou's.i 



divisions, Gammarina, Typhina, Lremudipnda, Epicarida, Cymo- 

 thoadse, Splioeroinaloda, Asellina, Idotoi'.a, and Oniscoda. 



De Haan, in his magnificent work upon the Crustacea of Jajian, 

 adopting the quinarian circular system of MXeay, divides the class 

 into five orders, — l)eciipoda,Stomapoda,Telradecapoda(£(/rio/>M«//rta, 

 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 LameUibranchige ; but thcadop- 

 of this, like any other sini;le character, has had the effect of 



