CONCERNING NATUKAL GROUPS. 161 



segments are far more numerous, and there is no definite 

 number of them, as in the two last-mentioned classes; the 

 absence also of jointed appendages and certain features con- 

 cerning their internal structure, lead some naturalists to rec- 

 ognize, under the name vermes, the worms and related 

 forms as a branch separate from the crustaceans and insects, 

 though still having certain features in common with them. 

 The name Articulates is therefore abandoned, and the crus- 

 taceans and insects are united in one branch, and called 

 ATthrojpods, a word derived from two Greek words, meaniag 

 jointed foot. The clams, oysters, mussels, snails, and some 

 other creatures which have not been mentioned in this book, 

 have certain essential features in common, and so they are 

 included in another great branch called MollusTcs, from a 

 Latin word, mollis, meaning soft, because the bodies of these 

 animals are soft. A very inappropriate name, because there 

 are many other animals which are soft bodied having no 

 relation with the shell-fish or Mollusca. 



146. ITow, 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-cha/racters, or typical cha/racters, and that animal which 



