CONCERNING NATURAL GROUPS. 161 



In worms, too, the segments are far more numerous, and 

 there is no fixed or definite number of them as in the crus- 

 taceans and insects. The name Articulates is therefore aban- 

 doned, and the crustaceans and insects are united in one 

 branch or sub-kingdom, and called Arthropods, a word de- 

 rived from two Greek words, meaning jointed foot, while 

 the worms are embraced under another sub-kingdom, Vermes. 

 The clams, oysters, mussels, snails, and the squids or cuttle- 

 fishes (a class of animals which have not been mentioned 

 in this book), have certain essential features in common, 

 and so they are included in another great branch or sub- 

 kingdom called Mollusks, from a Latin word, mollis, mean- 

 ing soft, because the bodies of these animals are soft, though 

 often protected by a hard shell. The name Mollusca as 

 applied to these creatures is very inexact, because there 

 are other animals, such as the worms, for example, which 

 are soft-bodied, yet having no relation whatever to the 

 shell-fish or Mollusca. 



146. Now, these divisions or branches not only include 

 animals which are simple in their structure, but animals 

 which are very elaborate. All the animals in each great 

 division, however, must embrace creatures that possess the 

 same essential characters. With a knowledge of these essen- 

 tial features, it has been customary to make a diagram of a 

 theoretical animal out of these characters only. This theo- 

 retical figure is called an archetype, meaning an ancient type, 

 or first type, and the characters composing it are hence called 

 type-characters, or typical characters, and that animal which 



