PHYLUM ARTHROPODA 301 



(4) Malacostraca. — The Malacostraca are, as a rule, 

 larger than the Entomostraca, and include the more familiar 

 Crustacea, such as crayfishes, lobsters, crabs, shrimps, and 

 sow-bugs. Some of them are aquatic, others are terrestrial, 

 and a few are parasitic. 



The order Isopoda contains a number of common Malacos- 

 traca (Fig. 220). Most of them are marine, but some live in 

 fresh water and on land. They are the largest group of terres- 

 trial Crustacea. The sow-bug, Oniscus, and the pill-bug, Arma- 

 dillium, live under stones, boards, and similar places that are 

 dark and moist. Although land animals, they breathe by means 

 of gills situated on the under surface of the abdomen. 



The Amphipoda are aquatic, except a few species which leap 

 about on the beach, and are called beach- fleas. Gammarus 

 (Fig. 221) is called the fresh- water shrimp. Talorchestia 

 (Fig. 216) is a sand-hopper common on sandy beaches between 

 the tide-marks. Caprella is a peculiar brown amphipod which 

 so closely resembles the seaweeds or hydroids among which it 

 lives that it can be detected only by an experienced eye. 



The mantis shrimps belong to the order Stomatopoda. This 

 common name was derived from their resemblance to the insect 

 called the praying-mantis (Fig. 270). They are exclusively 

 marine. Squilla empusa (Fig. 222) lives along the eastern coast 

 of the United States. 



The order Decapoda contains the lobsters, crayfishes, crabs, 

 and shrimps, and is the most important group of the Crustacea. 

 The name Decapoda refers to the fact that only the last five 

 pairs of thoracic appendages are used for locomotion. 



The lobster is of considerable economic importance. It is 

 most abundant along the Atlantic coast from Labrador to Dela- 

 ware Bay, and lives on the bottom from near shore to a depth 

 of one hundred fathoms. About fifteen million lobsters are sent 

 to market annually, and unless their capture is regulated, they 

 will soon be exterminated. Shrimps and prawns are also used 

 as food for man. Palcemonetes (Fig. 224) is a common shrimp 



