3222 THE JOINTED ANIMALS 



shield that covers it the carapace. Sometimes too, as in the crawfish, the anterior 

 three pairs of thoracic appendages are transformed into jaws, and on this account 

 are called the maxillipedes or foot jaws; and in such cases only the remaining five 

 pairs, called the trunk limbs, are large, and useful for locomotion or seizing prey. 

 In less highly organized 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 fore part 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 characterized 

 by having the posterior five pairs of thoracic limbs strongly devel- 

 oped, 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. Bach 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 aper- 

 ture 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, bail- 

 ing 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 cara- 

 pace above the bases of the limbs. 



SHORT-TAILED GROUP SUBORDER Brachyura 



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

 Macrura, 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 



