362 THE INVERTEBRATA 



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. 278). 

 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. 269; 201, 202, 211 G, 214, 216, 

 217, 220, 222, 223, 272), 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, which are inter- 

 mediate 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 Palinura (crawfishes 

 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 swimming by means 

 of flattened legs. The Brachyura proper are distinguished from other 

 brachyurous forms by the occurrence in nearly all the latter of well- 

 formed uropods, which the true crabs do not possess, and by a fusion 

 of the edge of the carapace with the epistome, a sternal plate which 

 lies in front of the mouth (see p. 367). 



As an example of the Brachyura we shall describe Carcinus maenas^ 

 the common shore crab of Britain (Figs. 270, 271, 273-277). The 

 depression to which is due the difference in shape between the 

 cephalothorax of this typical crab and that of a crayfish or prawn has 

 brought it about that in a transverse section (Fig. 271) the carapace 



