106 VERTEBRATA. 



the metacarpus cany the digits, which also vary in number, 

 but are composed each of from two to three cylindrical bones, 

 which are known as the 'plmlanges {])). 



Homologous parts are, as a rule, readily recognisable in 

 the hind-limb. The pelvic arch, l^y which the hind-limb is 

 united with the trunk, consists of three pieces — the ilium, 

 ischium, and ^'^^^^-s — which are usually anchylosed together, 

 and form conjointly what is known as the innominate hone 

 (fig. 483, i). In most Mammals, the two innominate bones 

 unite in front by a ligamentous or cartilaginous union, and 

 they constitute, with the sacrum, what is known as the 'pehis. 

 The hind -limb proper consists of the following parts : 1. 

 The thigh-bone or femur, corresponding with the humerus 

 in the fore-limb. 2. The bones of the shank, corresponding 

 with the radius and ulna of the fore-limb, and known as the 

 tihia and fibula. Of these, the tibia is mainly, or altogether, 

 concerned in carrying the foot, and it is thus shown to cor- 

 respond 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 carries the digits, which consist, each, 

 of from two to three small bones or iilialangcs, as in the fore- 

 limb. 



The sub-kingdom Vertebrata is divided into the followino- 

 five classes : — 



1. Pisces (Fishes). — Eespiration 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 exclu- 

 sively 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 oc- 

 cipital condyles. The limbs, when present, never converted 

 into fins, and composed of the same parts as in the higher 

 Vertebrates. 



