262 CLASSIFICATION OF ANIMALS 



562. TheMacrura natantia (swimming large tails) are generally com- 

 pressed laterally, the ambulatory appendages are slender and the animal 

 depends chiefly on the backward stroke of the strong abdomen for loco- 

 motion. The Macrura reptantia (crawling large tails) are not compressed 

 and the thoracic appendages are strong as is also the abdomen. The 

 section Anomura includes the hermit crabs which live habitually with 

 the abdomen thrust into an empty snail shell. For this reason the 

 abdomen is twisted, the posterior thoracic appendages reduced and the 

 caudal fin transformed into an unsymmetrical hold-fast organ. The 

 Brachyura are the true crabs in which the abdomen is much reduced and 

 turned forward under the cephalo-thorax. 



563. The Arthrostraca have sessile eyes and the thoracic shield is almost 

 or wholly wanting. In the Isopoda the body is flattened dorso-ventrally 

 and there are no gills on the thoracic appendages. The Amphipoda are 

 compressed laterally and gills are present on the thoracic appendages. 

 The body is curved with the abdomen turned down and forward. The last 

 appendages of the abdomen are turned backward and are used in connec- 

 tion with the backward stroke of the abdomen to spring the body forward 

 analogous to the movement of a flea, hence the name. Most of the 

 ^lalacostraca are marine. The crayfishes are fresh-water representatives 

 of the order. A number of crabs live on land though usually in the 

 vicinity of water. The wood-louse is also a familiar terrestrial form, 

 though it is also at home only in damp places. The terrestrial crabs have 

 rudimentary gills and the gill chamber takes on the function of a lung. 



564. Class n. Palaeostraca. — These arthropods have only 

 one pair of appendages anterior to the mouth and five pairs 

 surrounding the mouth/ Those around the mouth serve both 

 for the ingestion of food and for locomotion. 



565. Order 2. — The Xiphosiira have a broad horseshoe-shaped cephalo- 

 thorax, an unjointed abdomen and a long spine-like terminal appendage, 

 to which the name '* sword tail" refers. There are a pair of ocelli and a 

 pair of compound eyes on the dorsal surface of the cephalothorax. The 

 first pair of appendages are chelate but are not used in locomotion. The 

 following five pairs are locomotor but their basal joints are spiny and 

 serve for the trituration of the food. The abdominal appendages are 

 broad leaf-like structures which protect the "book gills" which are 

 attached to them. There is only one genus of Xyphosura, the horseshoe 

 crab, Limulus. 



