CHARTER] Xavi 
ARTHROPODA 
CHARACTERISTICS. — Bilaterally symmetrical Coelomata, with 
a chitinous exoskeleton. Their body is segmented hetero- 
nomously. The segments usually bear a pair of jointed 
appendages, those in the neighbourhood of the mouth are 
modified im connection with the prehension and mastication 
of the food. The nervous system consists of a brain or. 
supra-oesophageal ganglion, a ring round the oesophagus, and 
a ventral, usually segmented, nerve cord. A heart is 
typically present dorsal to the alimentary canal, blood enters 
it through a series of lateral ostia ; the coelom is reduced, and 
to some extent replaced by a haemocoel. The sexes are typi- 
cally distinct, and the pared genital glands usually open by 
paired ducts. Cilia are universally absent from the group, 
with the single exception of Peripatus. 
The Arthropoda may be divided into two large groups, 
according to the nature of the breathing organs: (i.) the 
BRANCHIATA, which breathe by gills and are typically aquatic ; 
and (ii.) the TracnEaTa, which breathe by tracheae or lune 
books, and are typically terrestrial. 
The Branchiata include but one class, the Crustacea. 
The Tracheata include four: (i.) the Prototracheata, (ii.) the 
Myriapoda, (iii.) the Insecta or Hexapoda, and (iv.) the 
Arachnida. The first three classes may be grouped together 
as the Antennata, and opposed to the last class, the 
Arachnida. 
