450 ZOOLOGY. 



some Amphibia which retain gill-slits throughout life this 

 character is not diagnostic of the whole class. A complete 

 distinction is, however, found in the fact that no Amphibia 

 possess dermal fin-rays like those of fishes ; endoskeletal 

 fin-rays are also wanting, the endoskeleton of the paired 

 limbs having another form, that of the terrestrial limb or 

 cheiropterygium ; this consists of a single proximal bone, 

 then two parallel bones, then several small bones, the 

 limb terminating in five digits which alone have some 

 resemblance to fin-rays. The median fin-membrane when 

 present has no fin-rays at all. The skin is usually soft, 

 moist, and glandular, and serves as an accessory organ of 

 respiration, having a large vascular supply, and dermal 

 scales are usually absent; but this is not a diagnostic 

 character, as dermal scales are present in the snake-like 

 Amphibia of the tropics called the Gymnophiona and in 

 some Anura there are bony plates in the skin of the back. 

 The membrane bones of the skull represent the scales of 

 the head in fishes. The heart is three-chambered, the 

 auricle being divided into two, but not the ventricle. A 

 post-caval vein is present, but this occurs also in Dipnoi. 

 The posterior cardinal veins disappear in the adult. In 

 the vertebrae bony centra are well- developed, and the 

 neural arches possess zygapophyses which are not found 

 in any fishes. 



The Amphibia include four orders : the Urodela, which 

 retain the tail throughout life and contain some forms in 

 which one or more gill- slits persist in the adult, and others, 

 like the common newts, in which the gill-slits are entirely 

 closed in the adult ; the Anura, of which the frog is an 

 example ; the Gymnophiona, which are elongated snake-like 

 burrowing forms destitute of both limbs and tail ; and the 

 Stegocephalia or Labyrinthodonts, extinct tailed Amphibia 

 of large size with well- developed membrane bones on the 

 skull, and dermal exoskeleton. 



CLASS: EBPTILIA. 



In Reptilia, Birds, and Mammals, the fish-like larval 

 stage of the life-history is wanting, or rather it is passed in 

 the egg or in the oviduct. The embryonic development 



