ARTHROPODA. 
REATURES that compose this great Branch of the Animal Kingdom were 
regarded by Cuvier as articulated animals having a symmetrical body, in that 
both sides w^ere equal. One of his four great Divisions embraced these forms, 
under the title Articulata. The bodies of these animals are characterized by a 
peculiar feature, the series of rings, of which the earth-worm is a simple example. 
The circulating system is represented just under the back by a long vessel, 
the heart, connecting with vessels that propel the blood over the system, and 
return it to the gills, or lungs. The stomach and intestine lie in the median line of the body. 
The nervous system has ganglia, or enlargements of the nervous cords. 
The Arthropoda have certain features in common : bi-lateral symmetry, one side being 
like the other ; rings, or articulating parts, segments, arranged one upon another, each ring — 
theoretically — bearing a pair of limbs, which are also jointed. The blood is usually colorless, 
yet in some instances yellowish, or red, or purple. The globules of the blood, however, are 
not colored, the coloring matter being held in the fluid itself. The alimentary canal is usually 
nearly straight. The eyes are usually confined in the head. 
Authors have formerly divided the Arthropoda into two classes, the Insects and Crusta- 
ceans ; but the places of some examples are so obscure they are held somewhat in reserve. 
The Horseshoe Crabs and Trilobites, Water-Bears, Sea-Spiders, and Linguatulina are now 
resting between the two classes, Insecta and Crustacea, where authors are inclined to believe 
they will ultimately find a permanent place. 
INSECTS; INSECTA. 
The Insects afford the first examples of the Articulata, i. e., the jointeci animals without 
vertebrae. Their bodies are composed of a series of rings, and they are separated into at 
least two and mostly three portions, the head being distinct from the body. They pass 
through a series of changes before attaining the perfect form ; and when they have reached 
adult age they always possess six jointed legs, neither more nor less, and two antennae, 
popularly called horns or feelers. 
In most instances their preliminary forms, technically called the larva and pupa, are 
extremely unlike the perfect Insect; but there are some in which, at all -events externally, 
they retain the same shape throughout their entire life. The whole of the growth takes place 
in the preliminary stages, so that the perfect Insect never grows, and the popular idea that a 
little Insect is necessarily a young one is quite incorrect. It is true that smaller and larger 
specimens occur in every species, but this difference in size is due to some external influences 
that have acted on the individual ; and we find large and small examples of an Insect, say a 
wasp, or a beetle, just as we find giants and dwarfs among mankind. 
Insects breathe in a very curious manner. They have no lungs nor gills, but their whole 
body is permeated with a net-work of tubes through which the air is conveyed, and by means 
