INTRODUCTION* 



The branch or phylum of the animal kingdom known as 

 Arthropoda or joint-footed animals, includes those segmented 

 organisms furnished with a jointed, chitinous exoskeleton, each 

 segment or somite of which is typically provided with a pair of 

 jointed appendages which are also covered with the exoskeleton 

 in like manner with the body proper. The segments of the 

 arthropod body are more or less unequally developed and in 

 many cases groups of them are united. Usually a greater or 

 less number of the pairs of appendages which were primitively 

 for the function of locomotion, have been specialized for the 

 performance of other functions. 



The major classification of the Arthropoda into two sub- 

 phyla is based upon the nature of the respiratory organs, the 

 BRANCHIATA being those which breathe by means of gills, or by 

 lungs or tracheae which are metamorphosed from gills, and the 

 TRACHEATA, those which breathe by tracheae or air tubes. Each 

 of these sub -phyla is divided into classes based upon the seg- 

 mentation of the body and the modification of the appendages. 



Under the BRANCHIATA two classes are recognized, the 

 CRUSTACEA and the ACERATA. The crustaceans are, with few 

 exceptions, aquatic animals breathing with gills, although a few 

 carry on respiration through the general surface of the body; 

 they possess one or two pairs of appendages, the antennae, in 

 front of the mouth, which are mainly sensory in function; back 

 of the mouth there are several pairs of appendages which are 

 more or less completely modified for serving the mouth, and 

 back of these others whose function is for locomotion, either for 

 walking^ or for swimming. Each appendage typically consists 

 of a basal joint, the protopodite, which is continued distally in 

 two jointed branches, the exopodite and the endopodite, each of 

 which may be variously modified. 



The. ACERATA are arthropods which are primitively branch- 

 iate, but in which the branchiae may function as gills or lungs, 

 or they may be metamorphosed into air tubes or tracheae which 

 penetrate the body. The body is divided into a cephalothorax 

 anteriorly and an abdomen posteriorly, the line between the 



*In the preparation of the following pages of the "Introduction" and "Classifica- 

 tion of the Trilobita," the chapter upon this group of organisms in Eastman's translation 

 of Zittel's Text Book of Paleontology, by Prof. C. E. Beecher. has been largely drawn upon. 

 The matter has been somewhat rearranged and changed but in some cases nearly the same 

 words have been used. The indebtedness of the writer to this work is here acknowledged. 



