xi PHYLUM ARTHROPODA 523 



Sub-Class II. Malacostraca. 



Highly-organised Crustacea, usually of considerable size, and 

 having, -except in one order, a thorax of eight, and an abdomen of 

 seven segments. The appendages are usually highly differentiated. 

 As a rule the mesenteron forms only a small portion of the adult 

 enteric canal, and there is a gastric mill. The renal organs are 

 antennary glands. The nauplius stage is usually passed through 

 in the egg, but there is a more or less complex metamorphosis. 



ORDER 1. PHYLLOCARIDA. 



Small Malacostraca approaching the Entomostraca in structure. 

 The body is enclosed in a large bivalved cephalic carapace. The 

 thoracic feet are leaf-like, the abdominal feet biramo-us. The 

 abdomen has eight segments and a pair of caudal styles. 



The principal genus is Ncbalia (Fig. 422). 



ORDER 2. SCHIZOPODA. 



Small shrimp-like Malacostraca having the thorax more or less 

 completely covered by a soft carapace. The thoracic appendages 

 are all biramous. The eyes are stalked. 



Including Mysis (Opossum-shrimp), Euphausia, &c. (Fig. 423). 



ORDER 3. DECAPODA. 



Malacostraca in which all the thoracic segments are united with 

 the head to form a cephalothorax usually covered by a carapace. 

 The three anterior pairs of thoracic limbs are biramous foot-jaws, 

 the posterior five pairs are walking legs devoid of exopodites. The 

 third maxillipedes are leg-like. The eyes are stalked, the gills 

 thoracic. 



Sub-order a. Macrura. 



The abdomen is usually larger than the cephalothorax, and is 

 commonly held in an extended position. There is usually a distinct 

 rostrum. The eyes are not enclosed in orbits. The antennules and 

 antennae are large, the antennules are not sunk in pits, and the 

 antennse have an exopodite or squame. 



Including (a) swimming forms Penceus and Palcemon (Prawns), 

 Crangon (Shrimp), Lucifer, &c. ; (/#) creeping forms ffomarus 

 (Lobster), Astacus, Astacoides, Paranephrcps (Fresh-water Cray- 

 fishes), Palinurus (Rock-lobster), Scyllarus, &c. ; (7) anomalous 

 forms, approaching the Brachyura Pagurus (Hermit-crab), Birgus 

 (Cocoa-nut crab) ffippa, &c. (Figs. 424 426). 



Sub-order b.<* Brachyura. 



The abdomen is shorter than the cephalothorax and is per- 

 manently flexed under it. The eyes are enclosed in orbits or 

 tubular cavities of the carapace. The antennules and antennae are 



