000 ARTICULATES. 



In most of the Articulates, the blood moves forward in one or 

 more large dorsal arterial vessels, from which side branches are 

 given off, terminating in various trunks that convey the blood 

 backwards to the dorsal vessel. The blood is more highly or 

 ganized, has a deeper color, and contains a larger quantity of 

 corpuscles and fibrin than in either the Radiates or Mollusks. 



Respiration is accomplished by organs which, in all cases, are 

 perfectly symmetrical in those of this class which, like the Crus 

 taceans, habitually live in water by means of branchiae or gills ; 

 in others, by means of trachea or air-tubes, which receive air by 

 certain lateral openings, called Stigmata, (Gr. dots or marks.) 

 In rare instances, there exist cellular cavities, analogous to lungs. 

 The ARTICULATES may be arranged into the following classes: 

 I. INSECTA, Insects ; II. MYRIAPODA, Thousand-legged Worms, 

 &c. ; III. ARACHNIDA, Spiders, &c. ; IV. CRUSTACEA, Crabs and 

 Lobsters; V. CIRRHOPODA, Barnacles; VI. ANNELIDA, Anneli* 

 dans or Worms. (Plates XV. and XVI.) 



FIFTH PART. ENTOMOLOGY. (Gr. IVro.aa, entoma, an insect; 

 , logos, a discourse.) 



First class of ARTICULATES. Insecta, (Lat. from inseco, to cut 

 into.) INSECTS. 



The name given to this class refers to the divided structure 

 of the body or trunk of the animals which it includes. This is 

 generally composed of thirteen (sometimes fourteen) sections, of 

 which one forms the head; three the intermediate thorax, and 

 nine the abdomen. The head of a perfect insect has usually 

 three pair of jointed appendages. The first pair are called 

 antenna or feelers. They are affixed to the sides of the head 

 for the most part, between the eyes and the mouth, and have 

 from one to sixty joints or articulations. 



All true insects have six jointed or articulated legs, attached to 

 the thorax, and, usually, two or four win^s, situated upon its 

 three rings. (Plate XV. fig. 12.) The abdomen, which is fur 

 nished with many rings, contains the digestive organs. The 

 breathing process is accomplished by means of spiracles or pores, 

 on the side of each ring, for admitting the air, which is thus 

 made to permeate the whole body. 



Insects have a circulating apparatus, of which the central 

 organ, corresponding to the heart or the aorta in the higher orders 

 of animals, is a vessel or tube running beneath the skin of the 

 back, from which the white and cold blood is distributed in tis- 



