HIGHER CRUSTACEANS (MALACOSTRACA) 831 



the body is distinctly divided into two parts: the anterior, thorax 

 or trunk (th), comprising the first eight somites; the posterior, 

 abdomen (abd) , with the six following (to which the telson is added) . 



The appendages of the thorax are called thoracic limbs. Some 

 or all of the first three of them are in many cases specialized as 

 maxillipeds (maxp), and in this case the following five are called 

 peraeopods (perp) . The abdominal appendages are called pleopods 

 (pip), but those of the last (sixth) pair are often differentiated in a 

 peculiar way, so as to form with the telson a caudal fan, and in 

 this case the name uropods (urp) is used for them. 



The detail-structure of the appendages of the different regions 

 of the body is very different. The eyes (only doubtfully regarded 

 as appendages) may be entirely sessile, or may be elevated upon 

 short, subcylindrical, more or less movable eye-stalks. The an- 

 tennulae have an articulated base, with one or two terminal, articu- 

 lated branches (flagella). The antennae have an articulated basal 

 part, with one terminal, articulated flagellum, and often the basal 

 part has a lateral scalelike process: the antennal scale or scapho- 

 cerite. 



The mandible consists of a more or less solid part, to which an 

 articulated palpus may be attached. The maxillae are of various 

 shapes, and are probably to be regarded as modified anterior 

 thoracic appendages. They consist of an inner and an outer 

 branch (endopodite and exopodite), which, however, are often 

 augmented by certain parts belonging originally to the gill appa- 

 ratus. 



The most marked difference is between the thoracic and the ab- 

 dominal appendages. The former consist originally of a larger, 

 seven-jointed inner branch (endopodite), and a smaller, articu- 

 lated outer branch (exopodite), but the latter may be absent. 

 The seven joints of the endopodite are rather constant, although 

 some of them may become united, or others may be subdivided. 

 They have received separate names, which are, from the proximal 

 to the distal end: coxa, basis, ischium, merus, carpus, propodus, 

 dactylus (or coxopodite, basipodite, etc.). In certain thoracic 

 limbs, the last two joints (propodus and dactylus) assume a pecu- 

 liar position, forming a chela (pinchers, claws). 



