328 CRUSTACEA 



forms live on the land, but all others are aquatic animals. The majority 

 of these live in the sea, the Phyllopoda being the only order which is 

 better represented in fresh than in salt water. Crustaceans feed largely 

 on decaying animal and plant substances. Many are parasitic, especially 

 among the Copepoda, Cirripedia, and Isopoda. The barnacles are the 

 only sessile crustaceans. 



History. — Crabs and other decapods have been known and used for 

 food from time immemorial. They were first described by Aristotle who 

 calls them Malacostraca or soft-shelled animals in contradistinction to the 

 hard-shelled mollusks. Linnaeus placed them among the Insecta aptera. 

 The lower crustaceans were seen by the earlier microscopists, but very 

 little studied or understood until the time of 0. F. Miiller, who in 1785 

 brought together a large number and called them Entomostraca, or insect- 

 like crustaceans. Cuvier, Latreille, and Lamarck in the first years of the 

 new centuiy introduced the term Crustacea to include all crustaceans, 

 although the term had already been used as a synonym of Malacostraca. 

 The creation of the various orders of crustaceans is largely due to La- 

 treille, who introduced the names Branchiopoda, Isopoda, Ampliipoda, 

 Decopoda, and Phyllopoda. Milne-Edwards formed the order Copepoda, 

 and Burmeister introduced the terms Arthrostraca and Thoracostraca. 



American crustaceans have attracted many able investigations from 

 the time of Thomas Say in the first quarter of the last century to the pres- 

 ent time. In 1852 appeared the Crustacea of the Wilkes Expedition, by 

 James Dwight Dana, which was one of the most important zoological 

 works of the day. This and the works of Say, Stimpson, S. I. Smith, 

 and others form the groundwork of our present knowledge of American 

 forms. 



The class contains about 16,000 species, gi'ouped in 2 subclasses. 



Key to the subclasses of Crustacea: 



fli Small, often minute crustaceans without abdominal appendages. 



1. Entomostraca 

 Cj Larger crustaceans usually with abdominal appendages. . .2. Malacostraca 



Subclass 1. ENTOMOSTRACA. 



Small crustaceans, the majority of which are under a centimeter in 

 length; somites variable in number; head, thorax, and abdomen usually 

 distinctly marked, but in many the head and one or more thoracic somites 

 are fused together, forming a cepbalothorax ; body either elongate \vith 

 distinct seoTnentation or much shortened and enclosed in a chitinous shell 

 called the cara]^ace; parasitism has produced great changes in the form 

 of many entoniosti'aceans so that all semblance of the crustacean fonu is 

 often lost; appendages confined to the head and the thorax, 5 pairs being 



