ARTHROPODA 289 



The Arthropods have a bilaterally symmetrical body, 

 formed of a series of segments, but the segments are not 

 all alike, and some are fused together. Some, or all of 

 the segments, bear a pair of jointed appendages or limbs, 

 those near the mouth being modified to serve as jaws. 

 A chitinous exoskeleton is always present, and is often 

 strengthened by the deposition of carbonate or phosphate 

 of lime ; between the segments the integument remains 

 soft and flexible, so that movement of the parts of the 

 body is rendered possible. A heart is found in most 

 forms ; it is placed dorsally, and is provided with paired 

 slits, termed ostia. The body-cavity contains blood. In 

 some forms respiration takes place by means of the general 

 surface of the body ; others are provided with special 

 organs — gills (or branchiae), tracheae, or lung-books. The 

 gills are generally thin projections of the skin borne by 

 some of the appendages ; the tracheae are long, branching 

 tubes, filled with air, which penetrate all parts of the body 

 and open to the exterior; the lung-books are chambers 

 containing leaf-like folds of the skin. The nervous 

 system consists of a supra-oesophageal ganglion or brain, 

 connected by a ring round the oesophagus, with a ventral 

 cord, usually provided with ganglia, and placed beneath the 

 intestine. The sexes are separate in the majority of forms. 

 The Arthropoda are divided into the following Classes: — 

 (1) Crustacea, (2) Onychophora, (3) Myriapoda, (4) In- 

 secta, (5) Arachnida. The Onychophora include one 

 genus only — Peripatus, which has not been found fossil. 



CLASS I. CRUSTACEA 



The Crustacea are mainly aquatic animals, and are 

 abundant as fossils; they breathe generally by means of 



w. p. 19 



