130 



A COURSE ON ZOOLOGY. 



ring. Farther back is the ganglionic chain, 

 properly speaking, composed of thoracic and abdominal 

 ganglia, all united together by commissures. This whole 

 chain is, like the suboesophageal ganglion, placed under 

 the intestinal canal so that the cephalic ganglion alone 

 is above the digestive apparatus. 



The organs of sense are pretty well developed in the 

 articulates, and it is even possible that these animals 

 possess certain senses different from ours, and therefore 

 such as we can understand only very imperfectly. 



To review, the articulates or arthropods are dis- 

 tinguished by bodies composed of rings carrying pro- 

 cesses formed of mobile articles, and by their ganglionic 

 nervous systems, arranged in a ventral chain that is 

 connected with a cerebral mass by an ossophageal ring. 



The division of articulates comprises four principal 

 classes : Crustaceans, Insects, Arachnida, and Myriapods, 

 The peculiarities of these classes is as follows : 



I. In Crustaceans respiration is branchial or cutaneous. 

 The thorax has five or seven pair of 

 FIG. 70. limbs. The crayfish, lobster, crab, 



prawn, and wood-louse are crusta- 

 ceans. 



II. Insects have tracheal respiration ; 

 that is effected by means of tubes, 

 called tracheae or spiracles, kept open 

 by a sort of coiled spiral thread, and 

 distributed over the body and in all 

 the organs. Insects always have three 

 pair of feet, never more nor less. Flies, 

 butterflies, moths, and beetles are in- 

 sects. 

 III. In the Arachnida respiration is sometimes tra- 



ENLARGED TRACHEA 

 OP AN INSECT. 



