VERTEBRATA. 247 



out. In the compact skulls of the higher vertebrates, 

 however, their places are approximately marked by 

 their foramina, i.e. the holes {hat. foramen, a hole) 

 through which they pass in the skull ; and hence they 

 may conveniently be studied at leisure in the dry 

 skull. 



Classes of the Vertebrata. The Vertebrata 

 are divided into five classes, the Fishes, Amphibia, 

 Reptiles, Bieds or Aves, and Mammalia. 



The Fishes are exclusively aquatic ; they therefore 

 have gills to breathe with, and retain them all their 

 lives. 



The Amphibia are the only vertebrates that have a 

 larval stage different from the adult. This larval 

 stage is fish-like, being provided with gills ; the 

 Amphibia and Fishes are therefore sometimes classed 

 together under the name of Ichthyo'psida (fish-like 

 animals). The adult stage, tho' popularly called a 

 "reptile," differs from one in having two occipital 

 condyles, i.e. joints, by which the skull is set on to 

 the top of the spinal column. 



The name Saueopsida is sometimes used to include 

 under one heading the Reptiles and the Bieds, 

 animals so different in appearance that, when the 

 name was given by Professor Huxley, no one had 

 previously thought of classing them together. They 

 agree, however, in the important particular of having 

 the skull set on by one condyle only ; and there are 

 fossil types which form links between the two, and 

 thus show that they are related. 



