CLASS ARACHNIDA. 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 feet 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 trachese 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 pijcnogonides). The number of simple eyes is four at 

 most. These arachnides form our second and last order, that 

 of Trachean Arachnides. 



vol. xitt, c c 



