Section II—External Anatomy
As with the underlying skeleton, many features of the external anatomy are highly modified in association with the highly specialized saltatory mode of locomotion. The most obvious features are that frogs have very short, wide bodies and very large hind limbs and lack a tail (Figure 6.7).
With respect to the shortness of the trunk and relative size of the hind limbs, frogs exhibit the most extreme specializations of any vertebrate. The forelimb, including the brachium, antebrachium, and manus, is typical in form and proportions to that of other tetrapods. In the hind limb, the femur and crus
FIGURE 6.7 External features of the frog.
are also typical, but the pes is extremely elongated, a characteristic due to the marked modification of two proximal tarsals (see page 118), which produces an additional functional segment. There are four digits in the manus. The pes has five digits, which are webbed, as in many swimming forms. The thumb hypertrophies in males during the breeding season to help hold the female during amplexus. Claws are absent, as in amphibians generally.
The skin is, as in many anurans, thin and highly glandular. These features are associated with the considerable degree of respiration through the skin in most anurans. The mouth is very large, but otherwise normal. The external nares are rather small, located anteriorly and close together on the dorsal surface of the snout. The eyes are fairly large and project out from the top of the head, but in preserved specimens they are often retracted and covered by the small eyelids, which are not independently moveable. A nictitating membrane is present (Figure 6.8a).
Posterior to each eye, the conspicuous and circular tympanic membrane represents the ear externally (Figure 6.8a). Actually, the membrane itself lies deep to the skin, and may be separated from it. As noted above, the membrane is supported by the annular cartilage (Figure 6.8b). Males are easily distinguished from females by the size of the tympanic membrane. In females it is about the same size as the eye, whereas in males it is much larger than the eye.
The cloaca is present posteriorly. However, due to the absence of a tail, it appears to be located somewhat dorsally.
(a) Tympanic membrane and nictitating membrane, right lateral view
(b) Columella with tympanic membrane removed, right lateral view
FIGURE 6.8 Auditory region of the frog in right lateral view.
Key Terms: External Anatomy
cloaca external nares eyelids
eyes
manus
nictitating membrane pes
skin
tympanic membrane