404 THE INVERTEBRATA 



Order DECAPODA 



Eucarida in which the exopodite (scaphognathite) of the maxilla is 

 large; three pairs of thoracic limbs are more or less modified as 

 maxillipeds, and five are " legs " ; there is usually more than one series 

 of gills, of which some (podobranchiae) stand upon the coxopodites 

 of thoracic limbs, others {arthrobranchiae) upon the joint-membranes 

 at the bases of the limbs, and others (pleurobranchiae) upon the sides 

 of the thorax; and a statocyst is usually present in the proximal joint 

 of each antennule. 



The Decapoda owe their name to the condition of the hinder five 

 pairs of thoracic limbs, which are adapted for locomotion, typically 

 by walking but sometimes by swimming. Often, however, as in the 

 crayfish, one of these pairs bears large chelae and is incapable of the 

 locomotory function: others may also be incapacitated for it, as, for 

 instance, the two small hinder pairs of the hermit crabs (Fig. 292) . 

 Only in some of the lower genera is there any vestige of the exopodite 

 upon these five pairs. 



This order contains the most highly organized crustaceans. Among 

 its members there is great diversity in the habit of body and in the 

 form of the appendages, but two principal types can be observed. In 

 the first or macrurous type the caridoid facies is in the main retained, 

 the body is long and subcylindrical or somewhat compressed, the 

 abdomen is long and ends in a tail fan, the appendages are usually 

 slender, and any of the legs may be chelate. An example of this type, 

 the common crayfish, Astacus (Figs. 283 ; 212, 213, 222 G, 224, 226, 

 227, 231, 233, 234, 286), is described in most textbooks of elementary 

 zoology. The second or brachyurous type — which is not confined to 

 the Brachyura sensu stricto but occurs independently in various 

 members of certain groups, known collectively as the Anomura, that 

 are intermediate between the macrurous divisions of the order and 

 the Brachyura — has the cephalothorax greatly expanded laterally and 

 more or less depressed, while the abdomen is reduced and folded 

 underneath the cephalothorax. In it the appendages are as a rule 

 shorter and stouter than in macrurous forms, and only the first pair 

 of legs has a true chela. 



The suborders Penaeidea (primitive prawns), Caridea (prawns and 

 shrimps), Astacura (crayfishes and lobsters), and Palimira (craw- 

 fishes and bear-crabs) are macrurous. They are for the most part 

 swimmers, though some of them, as the Astacura and Palinura, do 

 more walking than swimming. The suborders Anomura and Brachyura 

 are walkers, though some of the crabs have their own ways of swim- 

 ming by means of flattened legs. The Brachyura proper are dis- 

 tinguished from other brachyurous forms by the occurrence in nearly 



