ARTICULATA. 119 
which is followed by the abdomen, and this is either distinct or united in a 
single piece with the former. These animals are not subject to a pertect 
metamorphosis, but they have, in some cases, a partial one. They have 
neither wings, antenne, nor upper lip; the number of feet is eight, affixed 
to the sternum or lower side of the cephalothorax ; the mouth is provided 
with a pair of mandibles or chelécera which usually project well in front, a 
pair of jaws or maxillz supporting palpi, and a lower lip. The chelicera 
are considered by Latreille not to be equivalent to the mandibles of the 
Crustacea and insects, but to correspond to the internal antennz of the 
former, and modified into predaceous organs; a view which, if correct, 
would deny proper mandibles to these animals, although these organs have 
an important place in the Articulata as a whole. Each of the two maxille 
bears a palpus or articulated organ much like the ordinary feet, but smaller, 
and like the ordinary feet of the Crustacea. (See pl. 78, figs. 35-40.) In 
Scorpio (pl. 77, figs. 51, 52), the palpi are cheliform, and much larger and 
stronger than the feet, resembling the first pair of true feet in the crab and 
lobster. There is an affinity between Scorpio and the Xiphosura or genus 
Limulus, in the mandibles, which are cheliform, each ending in a pincer. 
The feet correspond to those of insects, the round basal portion or coxa 
being joined to the thigh or femur by means of a short interposed trochanter, 
the femur being followed by the ¢dia, and this by the tarsus, which has 
three articulations in the more typical forms. 
The eyes are simple and smooth, and their number varies from two to 
eight. Theirnumber and relative position are much used as generic characters. 
The respiratory organs are of two kinds. The first (which resemble those 
of the Crustacea) are formed of numerous internal gills or laminee contained 
in internal pouches, and answering to lungs. The second kind are com- 
posed of trachez or air tubes (as in insects), connected with two spiracles. 
Those with the former are termed pulmonary, and the latter tracheary 
arachnidams. Both kinds may occur united, and the Podosomata (Pyeno- 
gonum, &¢.), like some of the lower Crustacea, have no means of respira- 
tion except the skin. 
The abdomen is generally soft and more or less globular in form, and it 
bears the spiracles, anal and generative apertures (the latter being near the 
base beneath), and the spinnerets in those forms which spin webs. The 
skin is generally of a tough leathery texture, and the muscles are attached 
to its internal surface, thus assimilating it to the external skeleton of the 
Crustacea and insects. 
Most of the Arachnida feed upon fresh animal food, as insects, which 
they take alive, either in their nets, or by running or suddenly leaping upon 
them. Some, as the ticks, are parasitic upon various animals ; whilst others, 
as some of the mites, feed upon vegetable matter, being found in flour and 
figs. Some mites infest dried meat and cheese. 
The Arachnida are mostly oviparous; the young, upon leaving the egg, 
are active, and resemble the adult, although some have a pair of feet less, 
which are finally acquired after several moultings, for, like the Crustacea, 
the members of this class change their integument from time to time. 
323 
