86 ARTHROPODA. 



Order II. DECAPODA. 



Branchiae enclosed in a special cavity on each side of the 

 thorax. Five pairs of legs, the first pair didactyle. 



The branchiae consist of numerous thin plates placed closely 

 together in the form of long quadrangular pyramids, nine on 

 each side in the common crab, twenty-two in the lobster. The 

 digestive organs comprise a large stomach and a large many- 

 lobed liver. The heart is a contractile sac, with six main arteries. 

 The eggs, after leaving the ovaries, are carried under the abdo- 

 men of the female, generally until they are hatched. In some, 

 as in Gecarcinidae, metamorphosis takes place within the egg. 



The gnathites are composed of a pair of mandibles, two pairs 

 of maxillae, and three pairs of foot-jaws. There are five pairs of 

 feet; the first and occasionally the second and third pairs are 

 didactyle ; the last pair is rudimentary in Dynomene. The seg- 

 ments of the head and thorax are closely soldered together and 

 covered by the carapace. 



Ecdysis, or moulting, occurs annually or oftener, until the 

 animal ceases to grow. The muscles are previously subject to 

 active absorption to within a third of their natural size in order 

 to facilitate withdrawal. The animal escapes where the abdomen 

 is connected with the carapace, or the carapace is split down the 

 middle. Crabs begin to breed long before they attain their full 

 size. They and their allies are the scavengers of the seas. 



Milne-Edwards [1834] divided the Decapoda into three sec- 

 tions, two fairly natural, but the intermediate one very hetero- 

 geneous. In the first, Brachyura, the abdomen is small and folded 

 under the body ; in the third, Macrura, the abdomen is well- 

 developed, in general longer than the body, with natatory appen- 

 dages at the end. Anomura [forming, it is said, the passage 

 between the two] has, if we except Paguridag, little to distinguish 

 it from one or the other ; Claus suppresses the section, but places 

 Porcellanidas in the former. The Brachyura are further divided 

 by Milne-Edwards into four families, the characters depending 

 chiefly on the form of the carapace : thus, in the " Oxyrhinques " 

 the carapace is narrowed anteriorly ; in " Cyclometopes " broad 

 and rounded anteriorly; in " Catametopes " quadrilateral or 

 ovoid; and in "Oxystomes" orbicular, prominent anteriorly. 

 This is an artificial grouping ; but the tribes into which he has 

 divided them, and which are now ranked as families, are natural, 

 and, with few exceptions, are adopted by recent authors. [French 

 authors rank tribes as subordinate to the family.] 



