432 CHORDATA. 
CHAPTER XXV. 
CLASSES OF VERTEBRATA. 
The Vertebrata are naturally divided into the aquatic 
or predominantly aquatic types called Axamnia and the 
typically air-breathing terrestrial forms called Amuzota. 
The names are derived from the absence and presence 
respectively of an enveloping foetal membrane called the 
amnion. 
In addition the Axamuza always possess, at one time in 
their life, fins, gills and lateral line sense-organs. 
The Azxamnia have three classes— 
I. CYCLOSTOMATA. 
2. PISCES. 
3. AMPHIBIA. 
The Ammnioza also have three classes — 
4. REPTILIA. 
5. AVES. 
6. MAMMALIA. 
Crass I.—CyYCLOSTOMATA. 
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 
