162 FIRST BOOK OF ZOOLOGY. 



possesses most of these characters, in the plainest manner, is 

 called a typical animal. This mode of presentation applies 

 as well to smaller groups as to larger ones. Thus in the 

 crustaceans, the lobster and crawfish might be called typical 

 crustaceans, as being the types or representatives of the class, 

 while a barnacle would certainly not be looked upon as a 

 typical animal of this class, though belonging to it. In 

 the same way an insect without wings would not be looked 

 upon as a type of the insects, because one of the leading 

 characters of the class of insects is the possession of wings. 



In making systematic tables to show the relative grade 

 an animal occupies, the simplest groups may be placed 

 lowest in the list to indicate their inferior position. For 

 instance, if the arthropods were to be arranged in a sys- 

 tematic table, those which have no lungs, but gills instead, 

 would be placed lowest, because it has been found in other 

 classes of animals that oftentimes the young or immature 

 animal has gills which are afterward replaced by cavities for 

 the purpose of breathing air direct, and the immature animal 

 is regarded as less perfect, or lower in its organization than 

 the mature or adult form. Consequently the crustaceans 

 would be placed lowest in the scale. Then would come the 

 air-breathing arthropods, and lowest among these would 

 come the spiders, as the head is not specialized from the 

 thorax. Next would come the myriapods, as in these the 

 head is specialized as in the insects. And, finally, the true 

 insects would come highest, as here the legs are reduced to 

 three pairs, the head as well as the thorax is definitely sepa- 



