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CRUSTACEOLOGY. 



Cfiistseea. these changes' will meet the views of all those who are 

 '— "-/-""* competent to appreciate the true principles that should 

 regulate every philosophical arrangement. 



In the arrangement of the three classes Crustacea, 

 Arachnides, and Insecta, we have adopted certain al- 

 terations suggested by Mr Leach, of which we shall 

 give some account : He has proposed to take from the 

 class Arachnides, the orders, I. Tetracera, II. Myria- 

 poda of Latreille, and add them to the Crustacea ; and 

 also to take from the same class, the order Parasita of 

 the same author, and add it to Insecta, which, by this 

 alteration, will include all those animals having two 

 antennae and six legs : the Arachnides, by the same 

 improvement, will take in all that have no antennae ; 

 and, lastly, the Crustacea will comprehend the remain- 

 der. On this mode of arrangement we shall say no- 

 thing, except that it seems well adapted to facilitate the 

 progress of the student, and on tins ground appears to 

 deserve attention. 



The following are the characters of the class Crus- 

 tacea : 



Anatomical Character. 



Heart single; branchiae for respiration ; no vertebrae; 

 spinal marrow with many knots or ganglia ; muscles for 

 moving the feet. 



External Character. 



Body with naked jointed feet, formed either for 

 swimming or running ; no wings ; covering crustace- 

 ■ous, horny, or membranaceous, either shield-shaped, or 

 bivalve. Branchiae placed under the shelL 



This class is divided into three orders. 1 . Entomos- 

 traca. 2. Malacostraca ; and, 3. Myriapoda. The latter 

 was placed in the class Arachnides, by Latreille ; but, 

 as already mentioned, we are inclined to place it with 

 the Crustacea, for several reasons which we shall state 

 in their proper place. We now proceed to define the 

 Orders, Tribes, Families, and Genera of the class Crus- 

 tacea. 



ORDER I. ENTOMOSTRACA. 



Feet either branchial, or furnished with leaf-like 

 processes. Body, with a coriaceous or membranaceous 

 covering, which is either shield-shaped or bivalve. 

 Eyes, generally sessile or fixed; in some few peduncu- 

 lated, or placed on a footstalk. Palpi, double. Man- 

 dibules, obscure or wanting. 



In this order the antennae are sometimes wanting, in 

 some they axe very obscure, in others pencil-shaped, or 

 branched. The eyes are generally two in number, in 

 some distinct, in others united, so as to appear as one ; 

 the mouth, furnished either with jaws or a proboscis. 

 The mandibules without palpi. Maxillae or jaws, four 

 pr six. Feet, generally ten in number, formed for swim- 

 ming. Tail, furnished with lamellae or setae, and 

 sometimes with a sword-like process. 



Observation. Some of the animals of this order un- 

 dergo changes during their growth ; these peculiarities 

 willbe noticed when the individual species are described. 



TRIBE I. THECATA. 

 Shell, shield-shaped. 



Family I. Xiphosura. 

 The clypeus or shield double, completely covering 



the body ; the feet simple and unequal in size ; tail Crustacea 

 sword-shaped ; antennae scarcely visible ; mouth with '■""V""""' 

 mandibules. 



Genus I. Limuluf. Shell composed of two pieces; 

 mandibules double-jointed ; tail horny and sword-sha- 

 ped. 



Family II. Pneumonura. 



The clypeus single ; feet simple, and unequal in size ; 

 mouth with a rostrum ; tail fibrous, or leaf-shaped. 



Genus II. Caligus. No mandibules ; tail with two 

 filaments ; the anterior feet terminated by a hook, the 

 rest formed for swimming. 



Genus III. Binoculus. No mandibules ; tail with 

 two lobes ; the anterior pair of feet terminated by a 

 nail, the second pair conic, the rest formed for swim- 

 ming. 



Family III. Phyllopoda. 



The clypeus single ; all the feet furnished with leaf- 

 like fins ; tail fibrous or filamentous. 



Genus IV. Apus. Mouth with mandibules ; tail 

 with two setae ; the feet leaf-shaped. 



TRIBE II. OSTRACODA. 



Shell bivalve ; Eyes most frequently confluent. 



Family IV. Monophthalma. 



Eyes confluent, or running together so as to appear 

 but one. 



* Two eyes. 



Genus V. Lynceus. Head exserted ; antennae ca- 

 pillary. 



** One eye. 



Genus VI. Daphnia. Head exserted ; antennae 

 branched. 



Genus VII. Cypris. Head concealed; antennae 

 terminated by a brush. 



Genus VIII. Cythere. Antennae hairy ; head 

 concealed. 



TRIBE III. GYMNOTA. 



Shell without any covering. 

 Family V. Pseudopoda. 



Head closely united to the thorax ; feet obscure or 

 obsolete. 



Genus IX. Cyclops. One sessile eye implanted in 

 the front of the thorax. 



Family VI. Cephalota. 



Head large, and evidently distinct from the thorax. 



* Eyes sessile. 



Genus X. Polyphemus. One eye ; two branched 

 feet extending horizontally. 



Genus XI. Zoe. Two eyes ; rostrum longer than 

 the thorax, and perpendicularly placed. 

 ** Eyes pedunculated. 



Genus XII. Branchiopoda. Body filiform. 



ORDER II. MALACOSTRACA. 



Feet either formed for swimming or running, the 

 tarsus being furnished with a horny tail. Body, with 

 a calcareous covering; two moveable and pedunculated 

 eyes, (in the third Family the eyes are fixed.) An- 

 tennae in all the genera four, four double. Palpi at- 

 tached tO the MANDIBULES. 



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