432 CHORD ATA. 



CHAPTER XXV. 



CLASSES OF VERTEBRATA. 



The Vertebrata are naturally divided into the aquatic 

 or predominantly aquatic types called Anamnia and the 

 typically air-breathing terrestrial forms called Amntola. 



The names are derived from the absence and presence 

 respectively of an enveloping foetal membrane called the 

 amnion. 



In addition the Anamnia always possess, at one time in 

 their life, fins, gills and lateral line sense-organs. 



The Anamnia have three classes — 



1. Cyclostomata. 



2. Pisces. 



3. Amphibia. 



The Amniota also have three classes — 



4. Reptilia. 



5. AVES. 



6. Mammalia. 



Class I. — Cyclostomata. 



The Cyclostomata were at one time, like Amphioxus, 

 included in the fishes, but the important differences from 

 the latter necessitate a separate class. In many respects 

 they are the most primitive of all the Vertebrata, whilst, as 

 must of necessity be the case, they also exhibit a degree of 

 specialisation. 



In their external appearance they approach the fishes, 

 especially the eels and other elongated types, but the entire 

 absence of limbs is remarkable. There is no evidence that 

 Cyclostomata ever possessed these organs. Again, they re- 

 semble fishes in the presence of lateral sense-organs, a median 

 fin with fin-rays, and in their method of respiration by gills 

 situated upon gill-slits. There are no jaws nor other free 

 visceral arches, the deficiency being supplemented by a 



