THE CRUSTACEA 



Division 2. Peracarida. 

 Order 1. Mysidacea. 

 „ 2. Cumacea. 

 „ 3. Tanaidacea. 

 ,, 4. Isopoda. 

 „ 5. Amphipoda. 



Division 3. Eucarida. 

 Order 1. Euphausiacea. 

 „ 2. Decapoda. 



Division 4. Hoplocarida. 

 Order Stomatopoda. 



Introductory. — The Crustacea form one of the Classes com- 

 posing the Sub -Phylum Arthropoda, and include, besides the 

 forms popularly recognised as Crabs, Lobsters, Crayfish, Prawns, 

 Shrimps, Sandhoppers, Woodlice, Barnacles, and Water-fleas, a 

 multitude of related organisms which are nameless in common 

 speech. 



The Class presents so wide a range of structural diversity that 

 it is all but impossible to give, in a few words, a definition which 

 shall apply to all its members. Of the great majority it may be 

 said that they are Arthropoda of aquatic habits, breathing by gills or 

 by the general surface of the body, having two pairs of antenniform 

 preoral appendages, and having at least three pairs of postoral 

 appendages acting as jaws, the three corresponding somites being 

 coalesced with the head. But while these characters are found 

 in the more primitive members, actual or hypothetical, of all the 

 sub-classes and orders composing the Class, the more modified types 

 furnish exceptions to every statement of the definition. Thus, the 

 land-crabs and woodlice are not only completely terrestrial in their 

 habits, but are provided with special organs for aerial respiration ; 

 the preoral appendages may be modified for locomotor or prehensile 

 functions, or may be quite wanting ; and some or all of the mouth- 

 parts may be suppressed. The most extreme modifications are 

 found in parasitic forms, and some of these, such as the Rhizocej^hala, 

 have lost, in the adult state, almost every trace, not only of 

 Crustacean, but even of Arthropodous structure. In these cases, 

 however, the larval stages aflford indications of affinit}^, while less 

 specialised forms provide connecting links with the typical Crustacea 

 and leave no doubt as to the natural character of the Class as a 

 whole. 



Ilistorical. — In the Sijstema Naturae (12th edition, 1767), 

 Linnaeus placed most of the Crustacea then known in his Class 

 Insecta (equivalent to the sub-phylum Arthropoda as noAv under- 



