SUPPLEMENT 
ON TI1E 
CLASS ARACHNIDA*. 
The animals of this class, though approximating to the 
insects, and though always considered as such in popular 
language, differ from them, as maybe seen in the text,, by 
certain marked characters which it is unnecessary to repeat 
here. They were first separated under the above denomina- 
tion by M. de Lamarck. He, however, comprehends in this 
class all the apterous insects of Linnaeus, with the exception 
of the genera cancer , monoculus , oniscus , which compose his 
class of the Crustacea, and those of termes and pulex , which 
he reunites to the insects properly so called. 
The Arachnida, thus named from the principal and most 
numerous genus, that of the spiders, ( arachne in Greek) have 
at the external surface of the body apertures for the admission 
of air, or stigmata, in this respect approaching the insects, 
and departing from the Crustacea. But they have not, like 
the first, antennae and a distinct head. Their mandibles, or 
rather the pieces which replace them, are contiguous and 
advanced parallel to one another, in the direction of the length 
of the body ; the jaws, or the parts analogous thereto, are but an 
expansion of the first articulation of the haunches of the an- 
* We adopt the word Arachnida in the Supplement as the term most 
used by English naturalists, though it will he observed that the equivalent 
Arachnides is the word made use of in the text. — Ed. 
