CLASS AIIACIINIDA. 
385 
live parasitically on vertebrated animals. There are some, 
however, found only in flour, cheese, and even on divers 
vegetables. Those which are attached to other animals 
often multiply very considerably. In some species, two of 
their teet are only developed with a change of skin ; and in 
general it is only after the fourth or fifth moulting at most, 
that the animals of this class become proper for generation. 
DIVISION OF THE ARACHNIDES INTO TWO ORDERS. 
The first have pulmonary sacs, a heart, with very distinct 
vessels, and from six to eight simple eyes. They compose 
the first order, that of the Pulmonary Arachnides. 
The others respire by tracheae, and present no organs of cir- 
culation, or if they have any, this circulation is not com- 
plete. The tracheae are divided from their origin into divers 
branches, and do not form, as in the insects, two trunks, ex- 
tending parallel to each other through the whole length of 
the body, and receiving the air from its different parts, through 
numerous apertures or stigmata. Here we find very distinctly 
but two at most, situated near the base of the abdomen (none 
in the pi/cnogonides). The number of simple eyes is four at 
most. These arachnides form our second and last order, that 
of Trachean Arachnides. 
c c 
tol. xnr. 
