CRUSTACEA. 301 



Prawns), the first of these five ambulatory thoracic legs is commonly 

 the largest, and didactyle {Jig. 130, 3). They all consist of six 

 joints, called, as they recede from the trunk, coxa, trochanter, femuTy 

 tibia, tarsus, metatarsus : the latter is often prolonged into a sharp 

 stifi" claw ; but, in the swimming crabs, it is flattened and expanded 

 in certain legs : when modified as pincers, the tarsus is expanded and 

 prolonged into a finger-shaped process, against which the last seg- 

 ment can be applied like a thumb. Each leg has usually two append- 

 ages attached to its base, called the palp and flagellum : analogous 

 appendages are attached to most of the thoracic and cephalic articu- 

 lated appendages, which subserve as jaws. 



The tergal arc of the blended third and fourth cephalic segments 

 extends over all the thoracic segments in the macrourous and brachy- 

 urous Crustacea, and constitutes the broad carapace in the crab. In 

 most Macroura the thoracic shield is formed of the lateral or epimeral 

 elements of the fourth cephalic ring, which meet along the back, and 

 give way preparatory to the moult. The tergal elements of the tho- 

 racic rings are not developed in either crabs or lobsters ; when these 

 rings are exposed by lifting up the cephalo-thoracic shield, the epi- 

 meral parts alone are seen converging obliquely towards one another, 

 but not joined at their apices. The thoracic legs, besides serving for 

 mastication, prehension, and locomotion, usually support more or 

 less of the branchial apparatus, and certain pairs are perforated 

 by the generative ducts, except in the crabs, in which the broad 

 sternal arc supports the generative outlets. 



The sternal arcs of the thoracic segments {fig- 130) send inwards 

 certain processes, called apodemata, which include spaces for protect- 

 ing the abdominal nervous chords and the branchiae, and which give 

 origin to the muscles of the legs. 



The seven abdominal segments are always united by flexible 

 joints, and have no apodemata. In those Crustacea in which the 

 thorax and its cephalic shield are small, the abdomen is long, as in 

 the squillae {fig. 128, A, 1 — 7, in outline), lobsters, and shrimps, and 

 it characterises the great tribe of Podophthalma called Macroura ; in 

 those in which the thorax and its shield are greatly expanded, the 

 abdomen is very short, as in the crabs, and it characterises the tribe 

 hence called Brachyura. This alternating excess and arrest of 

 development of the opposite divisions of the trunk well illustrate the 

 law of organic equivalents. 



In a small intermediate group, called Anomoura, including the 

 PaguridcB, or Hermit-crabs, and the Galatheida, the abdomen 

 presents intermediate proportions, and in some an anomalous struc- 

 ture : the fifth pair of ambulatory legs, e.g., is attached to the first 



