ZOOLOGY. 535 



class of their circle, the Corals and Snails ; their " quasi " 

 coat of mail is therefore harder, often richer in calca- 

 reous ingredients, and, in addition to this, lies frequently 

 upon the thorax or branchia as a special testa, called the 

 scute or shield. 



Class 9. Trac/ieal, Alary Animals. 

 Annulate animals, whose branchiae have undergone a 

 partial conversion into tracheae and into wings, are the 

 Insecta proper or Flies. 



3263. A Worm with feet, tracheae and wings, is an 

 Eiitomon or Insect. 



3264. The first separation takes place in respect to 

 the three tegumental segments of the body, the abdo- 

 men, thorax, and head. All three are, in the Insects, 

 more separated from each other than in the Branchial 

 animals, and united together usually by a narrow tube ; 

 even in cases also, where they are connate with each 

 other, they are still easily recognized by respect being 

 had to size, form, or appendages. 



Every Insect is divided into three segments. In the 

 abdomen are the organs belonging to the Worm, such 

 as the intestine, and a fatty body which appears to be 

 an analogue of the liver, a dorsal vessel, tubular sexual 

 parts and air-tubes (tracheae), but nothing else. 



The abdominal feet now disappear entirely, and even 

 the number of thoracic feet diminishes, owing, doubtless, 

 to the production of wings. 



3265. The thorax alone is reserved for the limbs or 

 locomotive members. It never carries more than three 

 pairs of legs and two pairs of wings. Of the viscera, it 

 contains nothing but the oesophagus, while in the Crabs, 

 important intestinal organs and even the liver, are situ- 

 ated within its cavity. It is in Insects therefore nothing 

 else than a medium of support to the respiratory organs 

 which have become limbs. Hence the thorax never has 

 more than three rings, namely, one for each pair of 

 legs. The wings invariably stand upon its two pos- 

 terior rings. 



