CHAPTER X11 
PHYLUM ARTHROPODA 
Chief Classes—CRUSTACEA, PROTOTRACHEATA, MyRIOPODA, 
InsEcTA, ARACHNOIDEA, PALAOSTRACA 
More than half the known species of animals are included 
in the Arthropod phylum, for of insects alone there are said 
to be more species than of all other animals taken together. 
The Arthropods are in some ways like Annelids—in the 
bilateral symmetry; in the division of the body into suc- 
cessive segments, some or all of which bear appendages ; in 
the plan of the nervous system; and so on. Furthermore, 
Peripatus, which has air-tubes or tracheze somewhat similar 
to those of Myriopods and Insects, has nephridia like those 
of some Annelids ; and the biramose appendages of a simple 
Crustacean like Afus may be compared with the parapodia 
of an Annelid. 
It is difficult to discern the relationships of the various 
classes included in the Arthropod phylum. Crustaceans, 
most of which are aquatic and breathe by gills, are often 
opposed to the Prototracheata, Myriopods, Insects, and 
Arachnoids, most of which are terrestrial or aerial, and 
breathe by trachez, or possible modifications of these. 
Three divergent groups—the King-crabs (Zimu/us), and 
the extinct Eurypterids and Trilobites—may be conveniently 
referred to a separate class—Palzostraca. 
General Characteristics of Arthropods (to which primitive, 
parasitic, and degenerate forms present exceptions) 
The body ts bilaterally symmetrical, and consists of numer- 
ous segments variously grouped, Several or all of the segments 
