Modern Amphibians

Modern amphibians are a very diverse group, but they do share several key characteristics:

• Most change from an aquatic larval stage to a terrestrial adult form. This transformation is called metamorphosis.

• Most have moist, thin skin with no scales.

• Feet, if present, lack claws and often are webbed.

• Most use gills, lungs, and skin in respiration.

• Eggs lack multicellular membranes or shells. They are usually laid in water or in moist places and are usually fertilized externally.

Order Anura

Anurans (frogs and toads) are found worldwide except in polar climates and a few isolated oceanic islands. They live in a variety of habitats, from deserts and tundra to tropical rain forests. Many anurans spend at least part of their life in water, and some species are permanently aquatic. Many other species live and reproduce on land. Figure 40-4 shows two examples of anurans. The term toad is commonly used for any anuran that has rough, bumpy skin, as seen in Figure 40-4a. The term frog commonly refers to anurans having smooth, moist skin, such as in Figure 40-4b. These terms are general descriptions, however, and do not refer to any formal groups of anurans.

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figure 40-4

Anurans include toads and frogs such as the plains spadefoot toad (a), Scaphiopus bombifrons, which can be found throughout the United States, and the White's tree frog (b), Litoria caerulea, which is common in Australia.

Anurans are characterized by a body adapted for jumping. Long, muscular legs provide power for the jump. The anuran body is compact, with a short, rigid spine and strong forelimbs that help absorb the shock of landing. The word anuran means "tailless" and reflects the fact that no adult anuran has a tail.

Adult anurans are carnivores that feed on any animal they can capture. Some frogs have a sticky tongue that can be extended to catch prey. Many species of anurans return to water to reproduce. In nearly all species, eggs are fertilized externally. The fertilized eggs hatch into swimming, tailed larvae called tadpoles.

Order Caudata

Salamanders have elongated bodies, long tails, and moist skin. Except for a few aquatic species, they have four limbs. The smallest salamanders are only a few centimeters long, while the largest reach lengths of 1.5 m (4.5 ft). Like anurans, salamander species range from fully aquatic to permanently terrestrial. Terrestrial salamanders usually live in moist places, such as under logs and stones. Larval and adult salamanders are carnivores. They are active mainly at night. Figure 40-5 shows two representative salamanders.

Most salamander species live in North America and Central America. There are very few species in Africa and South America, several species are found in Asia and in Europe, and there are no species found in Australia. With more than 300 species, the lungless salamanders (family Plethodontidae) are the largest group of salamanders. As their name suggests, these salamanders lack lungs. They absorb oxygen and release carbon dioxide through their skin.

Like most anurans, many salamanders lay their eggs in water, and the eggs hatch into swimming larval forms. Other species can reproduce in moist land environments. Eggs laid on land usually hatch into miniature adult salamanders and do not pass through a free-living larval stage. Most salamander species have a type of internal fertilization by which females pick up sperm packets deposited by males. In some terrestrial species, the female stays with the eggs until they hatch, which can take up to several weeks.

Order Caudata Eggs Images

Word Roots and Origins caudata from the Latin cauda, meaning "tail"

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figure 40-5

The flatwoods salamander (a), Ambystoma cingulatum, and the spotted salamander (b), Ambystoma maculatum, are members of the order Caudata. A. cingulatum, which lives only in Florida, is an endangered species; A. maculatum can be found from eastern Canada to eastern Texas.

figure 40-6

Caecilians, such as Ichthyophis kohtaoensis (a) and Caecilia nigricans (b), are primarily carnivores. They are burrowing amphibians that are usually blind, and a few species have scales embedded in their skin.

Order Gymnophiona

The common name used to refer to members of the order Gymnophiona is caecilian (see-SIL-yuhn). Caecilians are a highly specialized group of legless amphibians that resemble small snakes, as you can see in Figure 40-6. Caecilians live in tropical areas of Asia, Africa, and South America. Caecilians average about 30 cm (12 in.) in length, but some species reach lengths of 1.5 m (4.5 ft). Because they have very small eyes that are located beneath the skin or even under bone, caecilians often are blind.

Caecilians are rarely seen, and little is known about their ecology and behavior. Most species burrow in the soil, but some species are aquatic. All species have teeth in their jawbones that enable them to catch and consume prey. They eat worms and other invertebrates, which they detect by means of a chemosensory tentacle located on the side of their head. All species are thought to have internal fertilization. Some species lay eggs, which the female guards until they hatch. In a few species, the young are born alive. These caecilians provide nutrition to their developing embryos. The young use their jaws and teeth to scrape secretions, called "uterine milk," from the walls of the female's reproductive tract.

figure 40-6

Caecilians, such as Ichthyophis kohtaoensis (a) and Caecilia nigricans (b), are primarily carnivores. They are burrowing amphibians that are usually blind, and a few species have scales embedded in their skin.

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