Dr. Leach’s Arrangement of the Crustacea, 4'0. SOJ 
A. Branchiis pro respiratione. 
Classis I. Ceustacea. 
B. Trachels pro respiratione. 
Classis II, Pedibus ultra 8. Capite distincto ; antennis 2, Myriapoda. 
Classis III. Pedibus 6 aut 8. Capite thoraceque coalitis ; antennis 0. Arachnides. 
Classis IV. Pedibus 6. Capite distincto ; antennis 2. Insecta. 
Classis I. CRUSTACEA, 
The Crustacea are arranged by Latreille into two orders, viz. 
Entomostraca and Malacostraca. These orders in my opinion 
should rather be considered as subclasses ; to the latter of which 
the Tetracera (placed by Latreille Avith the Arachnides) may be 
added, as they agree Avith the sessile-eyed Malacostraca in every 
essential point of internal organization. 
The Entomostraca are but little knoAvn, and consequently their 
arrangement is extremely imperfect. 
The Malacostraca have occupied a very considerable portion of 
attention, the result of which shall be given in the folloAving 
pages. 
Subclassis. MALACOSTRACA. 
I. PODOPHTHALMA. Oculi pedunculati. 
Ordo I. Brachyuka. Cauda brevis, inermis. 
Ordo II. Macroura. Cauda elongata, appendiculata. 
Legio II. EDRIOPHTHALMA. Oculi sessiles. 
Legio I. P O D O P II T H A L M A. 
o 
Ordo I. B RAC II YURA. 
Latreille arranges iXi^Brachyuraifeoxn the proportional bieadth 
and length of the thorax or shell) into two families ; but the dis- 
covery of genera unknown to that illustrious entomologist has 
convinced me that such a distribution is extremely unnatural ; 
2 s and 
Legio 
VOL. XI. 
