Adult Amphibians. External Anatomy

See Figures 3.1A, Band 3.2A, B within this chapter for generalized anatomic drawings of the skeleton of salamanders and anurans. See Figures 3.3, 3.4, 3.5, 3.6, and 3.7 for generalized anatomic drawings of adult specimens of the three orders of amphibians. See Plates 3.3, 3.4, 3.5, and 3.6 within the color section for photographs of the visceral anatomy of a caecilian, salamander, and frog.

Figure 3.1A. The skeleton of a typical salamander. Not all salamanders have all the skeletal elements depicted here, and the shape and arrangements of some skeletal elements may vary considerably. (John Gibb, Johns Hopkins University Department of Art as Applied to Medicine)

External Anatomy. The Adult Caecilian. The body form of caecilians strongly resembles that of annelid worms (e.g., earthworm, Lumbricus terrestris) due to the presence of cutaneous folds (primary and secondary annuli) in the skin, suggestive of a segmented body plan, and the absence of limbs. Coloration is variable, from slate grey to bright blue to yellow depending on the species.

The eyes are small and covered with skin in many species. The tentacle, a small olfactory and tactile sensory structure, is found in the nasolabial groove immediately ventral or rostral to either eye (Plate 3.7). Small external nares are present. Primary and secondary annuli create the ribbed appearance of the caecilian.

Some aquatic species may have a dorsal fin along the caudal third of the body. The cloaca is found at the terminus of the body, and there is usually little if any tail caudal to the cloaca. It has been reported that the sexes of some typhlonectiids and perhaps other caecilians may be externally distinguished by the shape of the cloacal opening, which has been modified in the male so as to enhance its grasping ability during mating. At this point in time, external sexing of caecilians by cloacal morphology is not reliable, for while it was reported that in the Rio Cauca caecilian, Typhlonectes natans, the male's cloaca is round and the female's is a longitudinal slit (O'Reilly et aI., 1995), this dimorphism has not held true in individuals necropsied at the Philadelphia Zoo.

Figure 3.1B. Skeletal nomenclature of a typical salamander. (Bradley Duncan Powell, Johns Hopkins University Department of Art as Applied to Medicine)

The Adult Salamander. The adult newt and salamander generally have four limbs, although the sirens only have forelimbs. Many species exhibit the derived trait of less than five toes per limb, and the one-toed amphiuma, Amphiuma pholeter, bears only a single digit per limb. Some salamanders, such as the hell-bender, Cryptobranchus alieganiensis, have light colored soles that should not be mistaken for scar tissue or other lesions (Plate 3.8).

Paired external nares and eyes are usually present, although cave-dwelling forms may lack eyes. Tympanic membranes are lacking in salamanders. Aquatic specimens mayor may not bear external plumate gills. Axolotls, Ambystoma spp., sirens, Siren spp., dwarf sirens, Pseudobranchus spp., mudpuppies and waterdogs, Necturus spp., the olm, Proteus anguinis, and blind salamanders, Typhlomolge spp., are aquatic species that possess external gills, while hellbenders, Cryptobranchus spp., Asian giant salamanders, Andrias spp., amphiumas, Amphiuma spp., and newts (family Salamandridae) lack these external structures. A tail is present, and in some forms the tail has cleavage planes to allow autotomy as an antipredator defense. In some plethodontid salamanders an obvious constriction ring is present at the tail base immediately caudal to the cloacal slit, and this serves as the site of tail breakage.

Figure 3.2A. The skeleton of a typical anuran. Not all anurans have all the skeletal elements depicted here, and the shape and arrangements of some skeletal elements may vary considerably. (John Gibb, Johns Hopkins University Department of Art as Applied to Medicine)

Many male salamanders may have swollen cloacal lips during times of reproductive activity, whereas female salamanders will show little or no increase in size of the cloacal lips at any time of the year (Plate 3.9).

During the breeding season nuptial pads may be present on the forelimbs of some salamanders, such as the Spanish ribbed newt, Pleurodeles waltl, and the hindlimbs of others, such as the Eastern red-spotted newt, Notophthalmus viridescens. Dorsal crests develop in males of Triturus spp. during breeding season. (See also Section 22.3, Sexual Dimorphism.)

Costal grooves lend a ribbed appearance to many species and are an important trait needed to identify many species. In some species (e.g., crocodile newt, Tylototriton shanjing) sharp points of the actual ribs may break through the skin as a defensive reaction to handling. Large toxin-containing parotid glands are visible caudal to the eye in some species. (These are not to be confused with the parotid gland of mam-mals, which is a salivary gland. Parotid refers to a location "near the ear." To avoid confusion, the amphibian gland is sometimes called the "parotoid" gland.) Mental glands may be visible underneath the chin of males in some species during the reproductive season.

The Adult Anuran. The adult anuran has four limbs. In anurans that depend on saltatory locomotion, such as ranid frogs, the hindlimbs are much longer and more obviously muscled than the forelimbs, whereas in anurans that tend to walk rather than jump, such as dendrobatids, the leg proportions are more equal. Five digits are present on the hind limbs and four or five are present on the forelimbs. Male anurans may develop nuptial pads on the forelimbs during breeding season. (See Table 3.1 and also see Section 22.3, Sexual Dimorphism.)

Figure 3.2B. Skeletal nomenclature of a typical anuran. (Bradley Duncan Powell, Johns Hopkins University Department of Art as Applied to Medicine)

Paired external nares and eyes are present. Tympanic membranes are present except in a few species. Gills are not present, but modifications of the skin, such as the filaments of the hairy frog, Trichobatrachus robustus, and the loose folds of skin of the Titicaca water frog, Telmatobius culeus, are functional equivalents. A postmetamorphic tail, actually an outpocketing of the cloacal tissue rather than a true tail, is present only in a few species of Leiopelmatidae and serves as an intromittent organ.

The cloaca is found at the tip of the urostyle and may be somewhat dorsally oriented. The cloaca is easier to see when the anuran is in ventral rather than dorsal recumbency. Large parotid glands are visible caudomedial to the eye in some species and are particularly prominent in bufonids. Vocal slits containing the vocal sacs may be visible in the vicinity of the maxillary hinge.

Some of the external characters that may be dimorphic in anurans include skin color (e.g., golden Alajuela toad, Bufo periglenes), tympanic membrane size (e.g., bullfrog, Rana catesbeiana), presence/absence of nuptial pads (e.g., White's treefrog, Pelodryas caerulea), toe shape (e.g., dyeing poison frog, Dendrobates tinctorius), adult body size (red-eyed treefrog, Agalychnis caliidryas), size of certain anatomic structures (e.g., the cranium of the lowland Caribbean toad, Peltophryne lemur), presence/absence of modified skin structures (e.g., papillae of the male hairy frog, Trichobatrachus robustus, marsupium of the female marsupial treefrog, Gastrotheca spp., heavily granulated skin of the male warty toad, Bufo spinu[osus), presence/absence of vocal sacs (e.g., oak toad, Bufo quercicus), presence/absence of "spines" on the forelimbs (e.g., Rosenberg's treefrog, Hyla rosenbergi) or lips (e.g., Taosze spiny toad, Vibrissaphora boringii), presence/absence of tusks or mandibular odontids (e.g., tusked frog, Adelotus brevis), and the presence/absence of a tail (e.g., tailed frog, Ascaphis truei).