ANNULOSA : INSECT A. 233 



of the order having been found in the Carboniferous epoch. 

 From rocks of this age several species of Chilognathous Myria- 

 pods have been discovered. They belong to the genera 

 Xylobins and Archiulus, and have been placed in a special 

 family under the name of Archiulida. The occurrence of 

 air-breathing articulate animals (both Arachnida and Myria- 

 poda) in the Carboniferous period is noticeable, as being 

 contemporaneous with the earliest known terrestrial Molluscs. 



CHAPTER XXXIX. 



INSECTA. 



GENERAL CHARACTERS OF THE INSECTA. 



CLASS IV. INSECTA. The Insecta are denned as articulate 

 animals in which the head, thorax, and abdomen are distinct; 

 there are three pairs of legs borne on the thorax; the abdomen 

 is destitute of legs ; a single pair of antenna is present ; mostly, 

 there are two pairs of wings on the thorax. Respiration is 

 effected by trachea. 



In the Insecta the body is divided into a variable number of 

 definite segments, or somites, some of which are furnished 

 with jointed appendages, and the nervous and circulatory 

 systems are constructed upon essentially the same plan as in 

 the Crustacea, Arachnida, and Myriapoda. The head, thorax, 

 and abdomen are distinct (fig. 80), and the total number of 

 somites in the body never exceeds twenty. " Of these, five 

 certainly, and six probably, constitute the head, which pos- 

 sesses a pair of antennae, a pair of mandibles, and two pairs of 

 maxillae, the hinder pair of which are coalescent, and form the 

 'labium.' Three, or perhaps, in some cases, more, somites 

 unite and become specially modified to form the thorax, to 

 which the three pairs of locomotive limbs, characteristic of 

 perfect Insects, are attached. Two additional pairs of loco- 

 motive organs, the wings, are developed, in most insects, 

 from the tergal walls of the second and third thoracic somites. 

 No locomotive limbs are ever developed from the abdomen of 

 the adult insect, but the ventral portions of the abdominal 

 somites, from the eighth backwards, are often metamorphosed 

 into apparatuses ancillary to the generative function." 

 (Huxley.) 



