SKELETON. 



i6i 



SKELETON OF FROG. 



contain more elements. The palatines and vomer, and more rarely the parasphenoid, 

 may be armed with teeth, like the upper jaw ; but in the frogs and toads the lower 

 jaw is very generally toothless. In all cases the teeth are small, simple, and pointed ; 

 being adapted for holding, and not for masticating. The shoulder-girdle, which is 

 largely cartilaginous, is placed 

 very close to the head, and com- 

 prises the usual elements. Each 

 scapula, or shoulder-blade, has an 

 upper cartilaginous portion, ex- 

 tending inwards nearly to the 

 middle line of the back ; while in 

 the frogs each metacoracoid has 

 an inward cartilaginous expan- 

 sion, which may either meet or 

 overlap its fellow, and is of much 

 importance in classification. In 

 advance of the metacoracoid s is 

 another pair of transverse bars 

 commonly known as the precora- 

 coids ; while in front of these is a 



single median rod termed the omosternum ; the proper sternum, or breast-bone, 

 occupying a similar position behind the metacoracoids. In the fore-limb the radius 

 and ulna may be united, and the wrist cartilaginous ; the number of toes among 

 living forms never exceeding four, and being sometimes reduced to three. More 

 variation exists in the hind-foot, the number of toes in the long-tailed forms 

 ranging from two to four, whereas in the frogs and toads it is always five. Only 

 in a few frogs and newts are the toes furnished with claw-like nails ; in the 

 greater number of forms these being naked, although often connected by webs, and 

 sometimes carrying adhesive discs on the lower surface. 



In all Amphibians the brain is of a very low type, its component 



portions lying in a line one behind the other, without overlapping. 

 All possess the three chief organs of sense, although in some instances the eyes 

 may be very minute and covered with an opaque skin. In frogs and toads the 

 eye is large and very highly developed ; generally possessing two lids, of which the 

 lower one is larger and thinner than the upper, and more or less transparent. 

 Greater variation exists in the structure of the ear, which is simplest in the 

 tailed forms. The nose opens externally in a pair of nostrils situated near the 

 muzzle, and by another pair of apertures into the mouth ; the latter character 

 distinguishing Amphibians from the majority of Fishes. The tongue, which acts 

 only in the very slightest degree as an organ of taste, and is wanting in one group 

 of frogs, is generally well-developed and thick, filling the whole space between the 

 jaws, and being capable of a large amount of motion ; it differs essentially from 

 that of the higher Vertebrates in that it is affixed to the inner side of the front of 

 the lower jaw, with its tip pointing down the throat. 



All Amphibians lay eggs, which are generally although not 



invariably deposited in fresh water, and fertilised as they are 



Soft Parts. 



Development. 



