ARTHROPODA. 



REATTJRES that compose this great Branch op the Animal Kingdom were 

 regarded by Cuvier as articulated animals having a symmetrical body, in that 

 both sides were 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 Idng'uatulina 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, /. c., the jointea 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 antenna?, 

 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 



