THE JOINTED-FOOT ANIMALS 171 



number of legs than centipeds, each somite, with the excep- 

 tion of several coming directly after the head, bearing two 

 pairs of legs. They live in damp places and feed on living 

 or decaying vegetable matter ; they are entirely harmless. 

 Many have the power of coiling themselves up when dis- 

 turbed. The larvae, when first hatched, have few somites, 

 and but three pairs of legs. 



Definition of Arthropoda. The animals described so far in 

 this book belong to the phylum (see page 109) Arthrop'oda 

 (Gr. arthron, "joint"; pous (pod-), "foot"). 



The body of an arthropod is made up of bilaterally sym- 

 metrical somites arranged in a linear series. There is always 

 a head composed of from four to six fused or united somites. 

 The somites of the remainder of the body are grouped into 

 one region, the trunk (centipeds and millipeds) ; or into two 

 regions, thorax and abdomen (Hexapoda, some Crustacea). 

 The head is sometimes fused with the thorax, the abdomen 

 being a distinct region (Arachnida, some Crustacea). Ap- 

 pendages, wherever present, are jointed. Except in the mil- 

 lipeds they occur as a single pair on each somite. The 

 somites and appendages are covered with a chitinous exo- 

 skeleton ; in some members of the phylum the exoskeleton 

 is very hard and filled with carbonate of lime. 



The digestive tract extends nearly straight through the 

 body from the anterior end to the posterior end. The blood, 

 which is usually colorless, is carried through the body in a 

 partially complete system of vessels with a tubular, or 

 heart-like, pumping organ in the dorsal region of the body 

 cavity. 



Respiration takes place through gills (Crustacea), lung- 

 like sacs (Arachnida), or through an internal network of 

 tabes (tracheae) opening in two lateral series on the exte- 

 rior of the body (centipeds, millipeds, insects, and some 

 arachnids). 



