GENERAL CHARACTERISTICS. 
5 
anatomy of the centipede and lobster will be found to be equally discernible, if 
other prominent types of Arthropoda be examined. Differences of course will be 
found to exist; but, on the whole, the plan of structure that has been sketched is true 
for all the classes. For instance, in all of them, except the Centipedes and Millipedes, 
there is a tendency in the more specialised members towards an increase in size of 
the limbs in the front half of the body, accompanied by a corresponding dwindling 
of those in the hinder part. Thus a crab and a spider walk upon four pairs of 
legs placed just behind the head, and an insect upon three; and in the case of the 
insect the legs of the hinder region have entirely disappeared, while the larger 
number of them have similarly vanished in the spider and the crab. There is also 
a tendency in the higher members of each class for the ganglia of the nervous 
chord to lose their segmental arrangement, and to become concentrated together in 
one large mass, placed near the seat of the greatest muscular activity. Never¬ 
theless, underlying all the modifications of structure—however extensive these may 
be—there is a common plan of organisation which may be regarded as typical of 
the Arthropoda. This may be briefly sketched as follows. The long bilaterally- 
symmetrical body is divided into a series of approximately similar segments, each 
bearing a pair of similar and segmented limbs. These limbs are the organs of 
locomotion; but some of those at the front end of the body, where comes the 
mouth and the organs of vision, take on the function of jaws, and are used for seizing 
and masticating food instead of for progression. The nervous system consists of a 
double ventral chord with ganglionic enlargements in each segment, and the first 
ganglia of this ventral chain are connected by means of a chord on each side of 
the oesophagus with the brain, which is lodged in the head. The heart, lying 
above the alimentary canal—which runs from one end of the body to the other— 
consists of a series of chambers, one for each segment of the body, and is provided 
with arteries for the distribution of the blood, and with slits or ostia for receiving 
it back again. 
The Arthropoda are divided into the following classes, the chief characteristics 
of which are described further on—(1) Insects (Insecta, or Hexapoda); (2) Centi¬ 
pedes (Chilopoda); (3) Millipedes (Diplopoda); (4) Spiders, Scorpions, Ticks, etc. 
(Arachnida): (5) King-crabs (Gigantostraca); (6) Crustaceans (Crustacea); (7) Pro¬ 
to tracheata (Per qoaf as). 
It is possible, however, to group these into larger divisions. The insects, 
centipedes, and millipedes, for example, may be placed together as Tracheata, 
characterised by the possession of trachem and of a single pair of antennae. The 
Crustacea stand alone in having two pairs of antennae, and in breathing with 
gills. By means, however, of the extinct class of the Trilobites, they are 
connected with the king-crabs; and these in possessing only six pairs of well- 
developed anterior limbs, and in having no antennae, strikingly resemble the 
Arachnida. Peripatus is very peculiar, but shows signs of a distant relationship 
with the centipedes, although in many anatomical features it is not very far 
removed from the worms. 
Distinctive The term insect, although originally and, according to the mean- 
Characteristics ing of the word, correctly employed in a wide sense to embrace 
of insects. a |] £} lose animals in which the body is externally divided into a 
