4 ARTICULATED ANIMALS. 



whither it returns by a ventral canal, which is sometimes 

 double. In the final species, the heart, or dorsal ventricle, 

 is itself elongated, like a canal. All these animals have an- 

 tennaj, or articulated filaments, attached to the fore-part of the 

 head, generally four in number, many transverse jaws, and two 

 compound eyes. It is only in some few species that a dis- 

 tinct ear is to be found. 



The third class of articulated animals is that of the AeaCH- 

 NIDA, which, like a great number of Crustacea, have the head 

 and thorax united in a single piece, with articulated limbs on 

 each side, but the principal viscera are enclosed in an abdo- 

 men, attached to the hinder part of this thorax. Their mouth 

 is armed with jaws, and the head provided with simple eyes, 

 varying in number ; but they never have any antennae. Their 

 circulation is performed by a dorsal vessel, which sends 

 forth arterial branches, and receives venous ones from 

 them ; but their mode of respiration varies, some of them 

 having true pulmonary organs at the sides of the abdo- 

 men, and others receiving the air through trachea;, like 

 the insects. Both, however, have lateral apertures — true 

 stigmata. 



The Insects are the fourth class of articulated animals, 

 and at the same time the most numerous of the whole animal 

 kingdom. Some genera excepted, such as the Myriapoda, 

 where the body is divided into a great number of nearly equal 

 articulations, the insects have it divided into three parts ; the 

 head provided with antennge^ eyes, and mouth ; the thorax, or 

 corslet, to which the feet are attached, and wings, where there 

 are any ; and the abdomen, which is suspended behind the 

 thorax, and encloses the principal viscera. The insects which 

 have wings do not receive them until a certain age, and often 

 pass throvigh two forms, more or less different, before arriving 

 at the perfect state of the winged insect. In all their states 

 they breathe by trachese, that is, by elastic vessels which re- 



