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- 
tennae, 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 Arach- 
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 aimed 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 tracheae, 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 inserts have it divided into three parts ; the 
head provided with antennse, e) es, 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 through two forms, more or less different, before arriving 
at the perfect state of the winged insect. In all their states 
they breathe by tracheae, that is, by elastic vessels which re- 
