78 AKTICULATA. 



CLASS CRUSTACEA. 



In this class the skeleton has the form of an external crust or 

 shell, which covers even the antennae, hairs, jaws and teeth. In 

 the smaller species it is chitinons ; in others it is hardened by the 

 addition of carbonate and phosphate of lime. The normal num- 

 ber of segments is twenty-one, seven each for the head, thorax 

 and abdomen. But most frequently the anterior segments form 

 one piece called the cephalothorax, leaving the abdomen jointed, 

 and terminated by a tail-piece {telson). Respiration is by means 

 of gills, or by the surface of the body, and all crustaceans are 

 organized for life in the water, though many live chiefly on land. 

 They have two pairs of antennae. The locomotive appendages 

 are more than eight in number, and are usually borne by the seg- 

 ments of the abdomen as well as by the thorax. This is a very 

 large and varied class, and contains many orders, of which only 

 three are herein represented. The earliest forms were Trilobites. 



In general we may say that the Palaeozoic Trilobites are fol- 

 lowed by Lobsters in the Carboniferous, and by Crabs in the 

 Jurassic. The articulates which came latest are the Barnacles 

 and Hermit-Crabs, which began in the Cretaceous. The Ostra- 

 coda are represented from the Cambrian to the present time. 



ORDER DECAPODA. 



This order includes the Crustacea which possess only five pairs 

 of legs. They have the gills beneath the "carapace" which 

 covers the cephalothorax ; and the compound eyes borne upon 

 eye-stalks, whence they are called Podopthalmata. The order is 

 subdivided into Macrura (long-tail), as the Lobster; Anomura 

 (irregular-tail), the Hermit-Crab ; and the Brachyura (short-tail), 

 the crabs. The Macrourans abound in the Oolite and Cretaceous 

 strata, while the Brachyurans attain their maximum develop- 

 ent in Tertiary beds and modern seas. 



