440 LECTURE xvm. 



Mammal would then have gained its instruments for leaping, as the 

 caterpillar acquires its organs of flight, and the concomitant de- 

 velopment and metamorphoses of the organs of sense, of digestion, 

 and of generation, would have been closely analogous in both 

 animals. 



Class INSECTA. 



Body chitinous, articulated, with articulated and uncinated limbs : 

 head provided with jointed antennae : respiratory system tracheal. 



Sub-class Mtriapoda. 

 Metamorphosis, proceeding from the hexapod type to the acquisition 

 of a greater number of limbs. 



Order Chilognatha. (Gaily worms.) 



Two mandibles without palps, divided into two portions, with im- 

 bricated teeth ; covered by a lower lip composed of the confluent 

 maxillae. Genera Glomeris, Platyulus, Polydesmus, Julus. 



Order Chilopoda. (Centipedes.) 



Two mandibles with small palps ; a quadrifid lip, followed by two 

 pairs of foot-jaws ; the second dilated, terminated by a strong hook, 

 perforated for the discharge of a poisonous liquid. Genera Geopkilus, 

 Scolopendra, Lithohius, Scutigera. 



Sub-class Hexapoda. 



Metamorphosis attended with the reduction of a greater number of 

 legs to six, or not leading to the acquisition of a greater number than 

 six, and usually with the superaddition of wings. 



Mouth suctorial. 



Order Aptera. No wings. (Lice, Fleas.) ' 



A. Thysanoura. Abdomen furnished at the sides with move- 



able pieces, or terminated by appendages 

 fitted for leaping. Genera Lepisma^ Ma- 

 chilis, Podura. 



B. Anoplvra. Abdomen without appendages. Mouth com- 



posed of a retractile sucker, or bilabiate 

 with two hooked mandibles. Genera Pe- 

 diculus, Nirmus, Ricimis. 



C. Aphaniptera. Thorax with scales representing rudimentary 



wings. Mouth composed of two laucet- 



