120 ZOOLOGY. 
The nervous system is much concentrated, being principally composed of a 
large ganglion in the centre of the cephalothorax, before which are two others 
with branches to the eyes and mouth. The principal ganglion has branches 
to the abdomen and feet. In the scorpions, instead of the great central 
ganglion, there are two rows of small ones, each united by a nervous cord. 
The class is divisible into the three sections, Aporobranchia, for Pyeno- 
gonum, &c., in which there are no special breathing organs; Zrachearta, 
in which breathing is effected by means of tracheze; and Pulmonaraa, in 
which the gills resemble the leaves of a book, and are adapted to breathing 
air only. The first section contains one, and each of the others two orders. 
The orders are named from the characters furnished by the body. The 
Podosomata, Leach, have the feet and body much alike; the M/onomeroso- 
mata, Leach, have only one segment apparent; in the Adelarthrosomata, 
Westwood, the segments are uncertain; in the Polymerosomata, Leach, 
they are numerous, as in the scorpion; and in the Demerosomata, Leach, 
including the common spiders, the body is divided into two portions. 
OrverR 1. Poposomata. These animals are placed among the Crustacea 
by Milne Edwards, because they have not the organs of respiration of the 
Arachnida, but respire by means of the skin, ike some of the lower Crus- 
tacea, a character which some of the Hydrachnide have, although no one 
would pretend to remove them to the Crustacea on this account. In form, 
the animals of this order approach to the crustacean genus Cyamus (pl. 
78, fig. 24), although they have but eight feet like the Arachnida. These 
are long and slender, and composed of eight articulations, including the 
claw. The head, or rather the rostrum, is lengthened, and either cylindrical 
or conical, without appendages, and the mouth is terminal and tri-lobed. 
The thoracic portion can be distinguished from the snout, which character 
distinguishes them from the great mass of the Arachnida, and it is composed 
of four segments, followed by a small abdomen. There are four eyes upon 
an eminence, situated upon the upper surface of the first segment of the 
thorax, and as the snout does not, in the opinion of Erichson and Kroyer, 
comprise the whole head, this must be looked for in the first segment of the 
thorax, which often bears a pair of cheliform organs, corresponding, in the 
view of Latreille, to the cheliform mandibles of Scorpio. The female (and 
in some cases the male also) has attached to the first segment of the thorax 
a pair of appendages much like the feet (but much smaller in size, and 
without a terminal claw), used to support the bunches of eggs, and which 
may be considered maxille. As Erichson regards the segment next to the 
rostrum as the head, he names its feet a third pair of jaws, whilst Kroyer 
considers them feet, on the ground that the segment to which they are 
attached is made up of the posterior part of the head (including the eyes) 
united without a division to the anterior part of the thorax. 
The alimentary canal is straight, and it is peculiar in having lateral 
tubular branches penetrating far into the feet, which dispenses with the 
necessity of having a regular circulation. In the more typical Arachnida, 
examples are found of a stomach with branches, but they do not enter the 
feet. 
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