302 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



which constantly consists of five joints, known as the 

 ischio-, mcso-, carpo, pro-, and dactylo-po- 

 dit.cs. The pair next in front form the great " for- 

 ceps " or chelae, the propodite of which is produced 

 and articulated upon the dactylopodite, so as to form 

 a most efficient seizing organ. The six pairs next in 

 front form the Onathites, the modifications of which 

 have been already described in connection with the 

 organs of digestion (page 123). The two most ante- 

 rior appendages form the antennae and the anten- 

 miles ; in the former the exopodite forms a flattened 

 squame, and the endopodite is many -jointed ; in the 

 antenmile both endopodite and exopodite consist of a 

 number of joints. By some authors the eye-stalks 

 are regarded as representing the protopodites of an 

 appendage. 



The paired appendages of the Arthropoda take on 

 very various functions in different groups, and vary 

 considerably in number ; in the Phyllopoda, the 

 IVIalacostraca, and the Myriopoda, all the seg- 

 ments of the body bear appendages; in the Copepoda 

 and Arachnid a they are absent from the hinder 

 part of the body ; and in the insects (Hexapoda), 

 there are but three pairs of definitely constituted ap- 

 pendages behind the gnathites, and these are always 

 attached to the thorax. 



The appendages may be very simple, and may be 

 nearly all similar in function, as in Peripatus, 

 where the incompletely jointed appendages, provided at 

 their free end with their two-hooked claw, are nearly 

 all ambulatory in function ; one pair alone forming 

 gnathites, one oral papilla?, and the last of all the anal 

 papillae. In the Myriopoda, all behind the gnathites 

 are ambulatory in function ; in the Branchiopoda they 

 form branchial swimmerets, and, as in other Entomos- 

 traca, there are never more than three pairs of 

 gnathites ; in the Copepoda and Ostracoda the second 



