300 HISTORICAL GEOLOGY 



say, all the invertebrates which are provided with 

 jointed legs. They have well-developed organs of 

 touch, sight, smell, etc., and their internal anatomy 

 is highly complex. 



(a) Crustaceans. 



Animals, usually aquatic, the majority of which 

 are covered by a hard outside shell made of a 

 number of plates. Here belong the lobsters, 

 shrimps, etc., of to-day, and the even more impor- 

 tant group of trilobites which are now extinct. 

 An idea of the general appearance and great 

 variety of the trilobites may be gained from 

 Figures 333, 334, 344, and 361. 



(6) Air-breathing arthropods. 



The insects, spiders, scorpions, and centipedes are 

 common on the land surface to-day, but fossil 

 remains of them are scarce. The animals of the 

 land are apt to be left in positions where they are 

 less likely to be preserved as fossils than are those 

 which live in the water. 



(9) VERTEBRATES. 



The highest branch of the animals contains those which 

 have a backbone or vertebral column. Among these 

 are the fishes, frogs, reptiles, birds, and four-footed 

 beasts in general. The subdivisions of this great group 

 we usually recognize without difficulty. 



(a) Fishes. 



Fishes are the lowest of the more familiar verte- 

 brates and are the only group which inhabits the 

 water exclusively. They have poorly constructed 

 skeletons which, in some species, consist chiefly 

 of cartilage instead of bone. In brain power, 

 also, they are the least advanced. They swim 

 by means of fins and breathe water through gills. 



