VERTEBRATA. 305 



The bones of the shank, corresponding with the radius and 

 ulna of the fore-limb, and known as the tibia and fibula. Of 

 these, the tibia is mainly, or altogether, concerned in carrying 

 the foot, and it is thus shown to correspond to the radius, 

 whilst the fibula corresponds to the ulna. 3. The small bones 

 of the ankle, known as the tarsus, and varying in number in 

 different cases. 4. A variable number of cylindrical bones 

 (normally five), which are called the metatarsus, and which 

 correspond to the metacarpus. 5. Lastly, the metatarsus car- 

 ries the digits, which consist, each, of from two to three small 

 bones we phalanges, as in the fore-limb. 



The sub-kingdom Vertebrata is divided into the following 

 five classes : 



1. PISCES (Fishes). Respiration by means of gills ; heart 

 usually two-chambered ; an exoskeleton, in the form of horny 

 scales or bony plates, generally present; blood cold. Limbs, 

 when present, in the form of fins, or expansions of the integu- 

 ment supported by bony or cartilaginous spines or " rays." 



2. AMPHIBIA (Amphibians). Respiration at first exclusively 

 by gills, afterwards by lungs, either alone or associated with 

 gills. Heart of the adult three-chambered ; blood cold. The 

 skull connected with the vertebral column by two occipital 

 condyles. The limbs, when present, never converted into fins, 

 and composed of the same parts as in the higher Vertebrates. 



3. REPTILIA (Reptiles). Respiration aerial, by lungs, and 

 never by gills. Pulmonary and systemic circulations connected 

 together either within the heart or in its immediate neighbour- 

 hood. Heart of the adult three-chambered in most; rarely 

 four-chambered. Blood cold. Skull united to the vertebral 

 column by one occipital condyle. Exoskeleton in the form 

 of horny scales or bony plates, or both combined. 



4. AVES (Birds). Respiration aerial, by lungs, and never 

 by gills. Bronchial tubes opening on the surface of the lungs 

 into air-sacs. A greater or less number of the bones almost 

 always hollow and filled with air. The skull connected with the 

 vertebral column by a single occipital condyle. Heart four- 

 chambered ; the pulmonary and systemic circulations distinct, 

 and the blood warm. Epidermic appendages in the form of fea- 

 thers. Pectoral limbs in the form of wings. Animal oviparous. 



5. MAMMALIA (Quadrupeds). Respiration aerial, by lungs, 

 and never by gills. The terminations of the air-passages (bron- 

 chi) never connected with air-sacs. Heart four- chambered ; 

 the pulmonary and systemic circulations distinct ; the blood 

 warm. Skull connected with the vertebral column by two arti- 

 culating surfaces or condyles. Some part or other of the integu- 



