346 LECTURE XVI. 



The common or typical number of articulated legs, therefore, in 

 this class, is six ; disposed in three pairs developed exclusively from 

 the thorax. In the Crustacea we saw that the number was greater, 

 and that the limbs were developed from the abdominal as well as 

 from the thoracic segments of the trunk. Such is the case also with 

 the Myriapodous Insects ; but these breathe the air directly by means 

 of tracheae, not by gills, and they have only one pair of antennae : 

 they likewise manifest, as we shall find, the typical Hexapod character 

 in their larval state, — a period during which, as in the Cirripeds, 

 Epizoa, and Acalephae, the Myriapod shows more of its true nature 

 and is more in accordance with the common type than during its final 

 and oviparous stage. 



Taking, however, a survey of the tracheal air-breathing Articulata 

 under their mature condition, they present more important characters 

 in common than any which indicate an affinity to the gill-bearing 

 classes ; and we find them offering the same ground for a primary 

 division as the Crustacea did, viz., in the number of the segments of 

 the body. 



This number is constant and definite in the higher and typical 

 members of the group, in which it is neither more nor less than 

 thirteen : in the rest it exceeds thirteen, and is vai'iable. 



There is no distinction between thorax and abdomen in the indefi- 

 nitely-jointed division ; and all the segments, save the first and last, 

 support jointed limbs. The Myriapoda I regard as a group equiva- 

 lent in the tracheal Articulata to the Entoraostraca in the branchial 

 Articulata; and, like them, they are the lowest organised, and tlie 

 least numerous and varied of the two divisions of their primary group. 

 The Myriapodous sub-class is divided, according to modifications of 

 the mouth, into Chilognatha and Chilopoda, answering to the genera 

 lulus and Scolopendra of Linnaeus, who first detected these natural 

 divisions. 



The Hexapod Insects may be classified, — 



1st. According to the phenomena of their development ; 



2nd. According to the structure of their mouth ; or 



3rd. According to the nature of their wings. 



Agreeably with the first character they would be divided into, — 



Ametabola, or those that undergo no metamorphosis ; 



Hemimetabola, or those that undergo a partial metamorphosis ; and 



Metabola, or those that undergo a complete metamorphosis. 



According to the modifications of the tropin, instrumenta cibaria, 

 or oral organs, the Hexapod Insects are divisible, like the Myriapods, 

 into two groups, viz., the Haustellata or suckers, and the Mandibu- 

 lata, or chewers and biters. 



