AMPHIBIA. 439 



separate and widely apart with no operculum. The skeleton 

 is cartilaginous and the palatoquadrate is free from the 

 cranium. There is a spiral valve to the intestine. A cloaca 

 is present and the kidney is mainly a metanephros. 

 Development is purely embryonic, the egg has much yolk 

 and the young is not hatched till like the adult. 



The Elasmobranchii are marine. The sharks are mostly 

 pelagic and the skates and rays mainly littoral or katantic. 



Order III. — HolocephalL 



A small order formed to contain Chimcera (the King of the 

 Herrings) and its allies. They resemble the last order in 

 their cartilaginous skeleton and some other structural features, 

 but differ in having an operculum covering the gill-slits, a 

 protocercal tail and no cloaca. The palatoquadrate (upper 

 jaw) is completely fused to the cranium. 



The few genera are widely scattered, one being a deep- 

 sea type. 



Order IV. — Dipnoi, 



The Dip7ioi or mud-fishes differ from the other orders 

 of fishes in the possession of true lungs in addition to their 

 gills, and in the partial division of the auricle into two, thus 

 producing a three-chambered heart ; the nasal sacs have 

 internal nares. The paired fins are archipterygia, ix.^ a 

 central axis with rays on each side ; the caudal fin is pro- 

 tocercal. There is a spiral valve and a cloaca and the 

 skeleton is partly cartilaginous and partly bony. 



Like the more primitive of the Teleostomi, the Dipnoi 

 are freshwater forms and have a discontinuous distribution. 

 Ceratodus is found in Australia, Protopterus in the Nile and 

 Lep id siren in the Amazon. 



Class III. — Amphibia. 



The Amphibia form a transition class from the two pre- 

 ceding to the three following terrestrial classes. The frog is 

 about the most terrestrial of all the class. Gills, median 

 fins and lateral line sense-organs are found throughout life 



