INTRODUCTION. 



The articulate branch of the animal kingdom contains animals 

 composed of simple rings, more or less similar to each other, 

 which contain and support the organs necessary to animal life. 



According to the method in which this plan of structure is 

 exemplified, articulate animals are divided into three classes : — 



Body permeated by air vessels. Insecta. 

 Body without air vessels ; 



Thoracic region distinct from abdominal. Crustacea. 



Thoracic region not distinct. Vermes. 



The class Insecta are again divided by subordinate modifica- 

 tions in the plan of structure into three sub-classes : — 



Head, thorax and abdomen distinct, legs 6. Insecta. 



Head and thorax usually agglutinated, legs 8. Arachnida. 



Head distinct, legs numerous. Myriapoda. 



The first sub-class Insecta (genuina) alone occupy our attention 

 at present. 



In examining the transformations of those which pass from a 

 larval form, which is very different from the adult, we find that 

 the changes may be grouped as follows : — 



1. Greater concentration of the central organs, and diminution 

 of the number of external segments. 



2. Greater complication of the peripheral organs. 



3. Hardening of the integument. 



"We also find that certain forms, when hatched from the egg, 

 resemble in appearance the parent, and finally assume the charac- 

 ters of the adult after growth, and repeated changes of skin. In 

 others the individual emerging from the egg bears no resemblance 

 to the adult, but after growth, accompanied with several changes 

 of skin, passes into a condition in which a body similar to that of 



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