CRABS. 255 



account are called the maxillipedes, or foot-jaws; and in such cases only the 



remaining five pairs, called the trunk limbs, are large, and used for locomotion or 



seizing prey. In less highly organised forms, all the maxillipedes may be free and 



foot-like, as in the mantis-shrimps, or only the anterior pair, as in sand-hoppers, 



may act as jaws. The remaining six segments, forming the abdomen, are usually 



provided with six pairs of small two-branched limbs, and to the last of these 



segments there is articulated a single plate, or telson, while the limbs, or uropods, 



are generally of large size, and form with the telson the tail-fin. 



The Malacostraca are divisible into two series, the Podophthalmata, containing 



those in which the eyes are perched on movable stalks, and the Edriophthalmata 



containing those in which they are sessile, or if raised upon stalks not movable. 



The former are further distinguished by having the forepart of the body generally 



covered by a carapace ; in the latter some of the thoracic segments are movable 



and there is generally no carapace. 



The first order (Decapoda) of the stalk-eyed series is characterised 

 Decapods. . . . . 



by having the posterior five pairs of thoracic limbs strongly 



developed, and forming either walking or swimming legs, or prehensile pincers. 



The three pairs of maxillipedes are generally transformed into jaws ; but in some 



of the lowest forms, as shrimps, the third or last pair are long and limb-like, so 



that in reality there are six pairs of large thoracic limbs. The gills, which are 



attached to the sides of the cephalothorax and to the basal segments of its limbs, 



are concealed in a gill-chamber, formed by the lateral portions of the carapace. 



Each gill may be compared to a plume consisting of a central stem, to which is 



attached a number of delicate processes in the form of flattened plates or of 



filaments. The front aperture of the gill-chamber is closed by a movable plate 



called the scaphognathite, and attached to the second maxilla. During life this 



plate is in constant motion, baling out the impure water through the anterior 



opening, and thus compelling a flow of fresh fluid into the chamber through the 



openings at the hinder end of the carapace above the bases of the limbs. 



Short-Tailed Group, — Suborder Brachyura. 



Decapods are divisible into two suborders, the Brachyura, or short - tailed, 

 and Ma crura, or long-tailed group. The first-named suborder contains those 

 members of the order which may be called crabs. Here the abdomen, or so- 

 called tail, is small, and shorter than the cephalothorax, against the lower surface 

 of which it is usually tucked away. In the males it is generally narrow, and 

 bears only one or two pairs of appendages, but in the female it is broader 

 and is furnished with four pairs of limbs. In neither sex is its last segment 

 furnished with a pair of uropods forming the tail-fin. The lower surface of 

 the cephalothorax is generally broad and triangular, and the third pair of 

 maxillipedes are short and flattened, and form, when in contact, a plate completely 

 covering the rest of the mouth-organs. The group is divisible into five tribes, the 

 first of these being the Cyclometopa, or those with rounded foreheads. It includes 

 most of the commoner species, such as the edible crab (Cancer pagurus), and 

 shore-crab (Carcinus mcenas). The former belongs to the family Cancridce, 



