PHYLUM ARTHROPOD A 299 



thoracic, biramous legs, with caudal fin, and short cara- 

 pace covering only part of the thorax. Examples : Squilla 

 (Fig. 222), Gonodactylus. 

 (3) Entomostraca. — The Crustacea belonging to the En- 

 tomostraca are the Branchiopoda, Ostracoda, Copepoda, 

 and Cirripedia. They live in fresh water, in salt water, on 

 land, or as parasites on other animals. The enormous numbers 

 of these little creatures may be ascertained by counting the 

 specimens that are captured if a fine gauze net is drawn through 

 the waters of lakes or streams. It has been estimated that, on 

 the average, each cubic meter of water in the small Wisconsin 



Fig. 224. — Order Decapoda. Palamonetes vulgaris, a shrimp. (From 



Davenport.) 



lakes contains about 40,000 individuals, and that 160 billion, 

 weighing altogether about twenty tons, may exist at one time in 

 a lake of eighty square kilometers. Usually a lesser number are 

 present in the waters of streams. The ocean is likewise popu- 

 lated with billions of these minute Crustacea. 



These small Crustacea are of little if any direct economic 

 importance to man, but indirectly they are of considerable value, 

 since they form the chief food of many edible fishes. 



The Trilobita are extinct Crustacea which are known only 

 from their fossil remains. They are associated in the strata of 

 the earth's crust with the remains of Crinoidea (Fig. 148), 

 Brachiopoda (Fig. 126), and Cephalopoda (Fig. 191). The 

 best-known species, Triarthrus becki (Fig. 209) , is from the Utica 

 shales (Lower Silurian) of New York State. It has two anten- 

 nae and many biramous appendages. 



The Branchiopoda include the leaf-legged Crustacea (Phyl- 

 lopoda), and the water- fleas (Cladocera). The fairy-shrimp, 



