176 GEOLOGY. 



many live in lakes, rivers, and swamps, and some even 

 on the dry land. The substance of these animals is soft, 

 as seen in the common oyster, and hence the name, which 

 comes from the Latin mollis^ soft. But most of them 

 have hard shells, and so are called, in common language, 

 shell-fish ; though some, as the slugs, are wholly uncov- 

 ered. This class includes oysters, mussels, clams, snails, 

 slugs, nautili, cuttle-fishes, etc. The shells of some of 

 these animals have contributed largely to the formation 

 of the limestone of the earth, but they are more zoolog- 

 ical in their relations than the previous class. So widely 

 distributed have shell-fish always been, and so durable 

 are their shells, that their fossils are of great use to the 

 geologist in determining the relative ages of the rocks. 



259. Articulata. This sub-kingdom includes insects, 

 worms, the spider and scorpion tribe, and the crab tribe. 

 These varieties are represented in Fig. 96. You have 

 here animals of extremely various character in many re- 

 spects. Nothing, for example, could be more unlike 

 than a butterfly and a lobster. Yet all these varieties 

 agree in one thing in having a covering which answers 

 to them as a skeleton, in giving firmness to the body, 

 and furnishing points of attachment to their muscles. 

 This is very obvious in such animals as crabs and lob- 

 sters, but it is equally true of insects and worms. This 

 skeleton coat of mail, as it may be called, has commonly 

 a very manifest ring-like arrangement ; and, as the rings 

 are jointed with each other, the name articulata, coming 

 from the Latin articulus, joint, is given to this division 

 of the animal kingdom. The articulata are mostly zoo- 

 logical in their relations, and are only slightly geological. 

 They live in every element in the air, the earth, and 

 the water. Some are animal-eaters, some vegetable-eat- 

 ers, and some both, and they, in their turn, are the food 

 of other animals of various kinds. 



260. Vertebrata. The animals of this sub-kingdom 

 have an internal skeleton with a back-bone, so called in 



