156 GENERAL ZOOLOGY 



DEFINITION OF ARTHROPODA 



The animals described so far in this book belong to the 

 phylum (see p. 97) Arthrop'oda (Gr. arthron, joint; pous (pod), 

 foot). All but a very few members of this phylum are in- 

 cluded in the four classes, Hexapoda, Arachnid a, Myriapoda, 

 and Crustacea. 



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 (Myriapoda) ; or into two regions, thorax 

 and abdomen (Hexapoda, some Crustacea). The head is some- 

 times fused with the thorax, the abdomen being a distinct 

 region (Arachnida, some Crustacea). Appendages, wherever 

 present, are jointed or segmented, and occur as single pairs 

 on somites. The somites and appendages are covered with a 

 chitinous exoskeleton ; 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 colorless, is carried through the body in a partially 

 complete system of vessels with a tubular, or heart-like, pump- 

 ing 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 tubes 

 (tracheae) opening in two lateral series on the exterior of the 

 body (Myriapoda, Hexapoda, and some Arachnida). 



The nervous system generally consists- of a "brain," dorsal 

 to the gullet, two connectives passing one on either side of 

 the gullet, and uniting below to form a ganglion, from which 

 a double nerve-cord extends along the ventral body-wall to 

 the posterior end. Sense-organs characteristic of the phylum 

 are segmented tactile organs, and simple and compound eyes. 



