Jointed A nimals. 97 



one strengthened by the possession of a ring of exo- 

 skeleton consisting of two parts, a dorsal, and a 

 ventral half arch. The limbs are articulated on each 

 side, between the half arches, each segment possessing 

 one pair. These generally remain distinct, even when, 

 the segments, as often happens, fuse together, so that 

 the number of constituent segments can be very often 

 detected from the limbs, even when the body rings 

 are .united into a continuous shield. 



The bodies of arthropods are bilaterally symme- 

 trical. They are also remarkable for the absence of 

 cilia at all periods of life. 



Each limb consists of several joints, each having 

 an external chitinous exoskeleton, containing the 

 muscles which move it. In the simplest arthropods, 

 the limb consists of a basal segment bearing two 

 appendages, an outer and an inner. In the higher 

 forms the limbs are divided into five, seven, or more 

 joints. These limbs are used for various purposes, 

 becoming modified into feelers, water-bailers, jaws, 

 swimmers, pincers, or walking feet 



All arthropods have a circulatory system and most 

 of them possess a heart which is a dorsal tube, divided 

 by valves into successive chambers, but there are 

 rarely fine blood vessels. There is a distinct respira- 

 tory system, and a complex digestive canal, except in 

 parasitic forms, and they have all a symmetrical ner- 

 vous system, consisting of two ganglia, one above, the 

 other below the throat, followed by a chain of ganglia 

 in the ventral portion of the body. They are divided 

 into four great classes, i. Crustacea, including all 

 those that breathe by gills, as crabs, lobsters, &c. ; 



H 



