Phylum Chordata
Subphylum Vertebrata
Class Amphibia
Amphibia = Gr. amphi - both or double; bios = life

The Frog Life Cycle:

The 3900 spp. of amphibians are grouped into 3 orders:
1. salamanders (order Caudata or Urodela)
2. frogs and toads (order Anura or Salientia)
3. the secretive, earthworm-like tropical caecilians (order Gymnophiona or Apoda) - limbless amphibians

I. Comparison and contrast of terrestrial and aquatic habitats

Animals are mostly water; therefore, many structural and functional characteristics are necessary for survival on land. Amphibians have most of these characteristics that all land animals have which allow survival on land.

1. Difference in water content --aquatic environments always present (available).
- terrestrial environments -- availability of water (humidity amount of water is extremely variable. Less water overall.

2. Greater Oxygen content of air.
-aquatic animals spend more energy extracting oxygen from the water than do land animals from air.

3. Water is more dense -- provides buoyancy and allows for good support even though more difficult to move through than air.
-survival on land requires (for large animals) a good, strong skeleton for support.

4. Constancy of water temperature
- water has high heat capacity; does not change temperature much unless large amounts of heat are added or removed. Most aquatic animals, especially marine spp. do not have problems with the drastic temperature changes that occur on land.

5. Higher variety of habitats on land as opposed to aquatic environments.

6. Greater opportunities for breeding on land. Safe shelter for eggs is more readily available on land than in water.

II. Origin and relationships of Amphibians

Land animals must be able to support their own weight, resist drying, cope with rapid temperature changes, and extract oxygen from air .

Requirements for Land Survival

A. Lungs - absolutely necessary for respiration (self-supporting and internal) on land.

- gills, with their fine filaments and gill lamellae require water to prevent collapse and desiccation

B. along with lungs, a more efficient circulatory system is required; all fishes have a 2 -chambered heart which pumps blood through gills to get oxygen and from there to the remainder of the body before returning to the heart. This single circulation system is okay for aquatic animals whose bodies are surrounded by water from which oxygen can diffuse into some parts of the body. Also, aquatic animals are metabolically less active and therefore do not require a highly efficient circulatory system.

On land, however, thick skins are important to prevent desiccation but restrict diffusion of oxygen form the air. Also, many terrestrial animals have a very high metabolic activity. Therefore, a very efficient circulatory system (in addition to the presence of lungs) is needed to meet the demands of terrestrial life. All land vertebrates have a double circulatory system which allows oxygenated blood to be returned directly back to the heart before being pumped to the rest of the body.

Components of the double circulatory system:

1. Pulmonary circulation-supplies the lungs with oxygen and provides a short, quick and efficient way of getting blood from the heart to the lungs where it can "pick up oxygen", supply the lungs with oxygen as well, and then return to the heart so that oxygen can then be transported to the rest of the body.

2. Systemic circulation-supplies blood with oxygen to all of the body except the lungs.

C. Limbs for travel on land.

a) strong, bony endoskeleton for support.

b) very well developed muscle system for movement.

c) well-developed CNS for coordination of muscles used to move the skeletal elements and move the body.
The fossil record indicates that amphibians appeared abruptly in the record as fully-developed amphibians with all of the characteristics aforementioned present and very well developed:

a.) all have (had) lungs
b.) all have (had) 3 chambered heart for double circulation
c.) all have (had) very well-developed limbs
d.) all have (had) very well-developed skeletal system
e.) all have (had) very well-developed muscle system
f.) all have (had) very well-developed CNS

***** No transitional forms found in fossil record that would indicate any gradual appearance of these necessary, terrestrial vertebrate characteristics.

Amphibian Characteristics

Classification

Structure and Natural History
1. Caecilians:
members of an obscure order Gymnophiona (naked snake).

2. Salamanders, order Caudata (meaning tailed amphibian). Newts are also included in this order; they are a specific type of salamander; Least specialized of all amphibians; found in nearly all northern temperate and tropical regions; Most spp. are found in North America.

Size: Most of them are small.

The common North America salamanders are less than 15 cm long.

Some species are aquatic and never produce true lungs: the gills are the major respiratory organ for the entire life cycle. these aquatic salamanders have all other amphibian characteristics. They can exist for short periods out of the water; some aquatic spp. are rather large (approx. 1 meter); The Japanese giant salamander reaches lengths > 1.5 meters.

Salamanders have simple primitive limbs; most have 4 (tetrapod) but a few spp. have only 2 limbs.

Most salamanders are carnivorous preying on worms, small arthropods, and small mollusks; most eat only things that are moving.

***Their food is rich in proteins; therefore, they produce lots of nitrogenous wastes, usually urea which is less toxic and requires less water for excretion.
*** They use the carbon skeletons of the protein-derived amino acids for their catabolic (energy) and anabolic (biosynthetic needs); Therefore, they do not store much fat (lipid) or glycogen (storage form of glucose, similar to starch;
*** Basically, the animal form of starch, less branched, is mostly a straight chain of polymerized alpha-glucose).
***Salamanders are ectotherms - derive heat from surroundings(external heat sources). Therefore, they have a low metabolic rate.

Breeding:
Terrestrial salamanders live in moist places under stones and rotten logs, usually not far from water. They do not show much diversity in regard to breeding habitats, due to the somewhat restricted habitats in which they are found. Fertilization of eggs is internal. The female picks up a packet of sperm Spermatophore) that has been previously deposited by the male on a leaf or stick. Aquatic species lay their fertilized eggs in small, group-like clusters under logs or in holes of soft dirt. Many spp. remain near eggs to guard them. The embryos of salamanders hatch from their eggs resembling their parents. The larvae do undergo metamorphosis during development, but not to the extent that tadpoles of frogs and toads do. American newts often have a terrestrial stage interposed between the aquatic larvae and the aquatic, breeding adults.

Respiration
All salamanders hatch with gills, but during development, they are lost in all except aquatic forms or in some spp. which do not undergo complete metamorphosis. Terrestrial salamanders have well-developed, fully functional lungs. Some salamanders have neither lungs nor gills and respire through their skin. The skin contains an extensive vascular network of capillaries which allow for gas exchange to take place just below the epidermis. This type of respiration through the skin is called Cutaneous respiration. Cutaneous respiration is facilitated by pumping air in and out of the mouth where further respiratory gas exchange can take place. The buccal (mouth) cavity has a highly vascularized membrane system that supplements cutaneous respiration in lungless, gill-less salamanders.

Order Anura

Frogs and toads; very specialized order of amphibians; very popular for educational purposes, but are actually poor representatives of the vertebrate body plan; why?

Classification, taxonomy, etc.

Frogs and toads are divided into 21 families; the most well-known North American families are:

The term toad is used rather loosely to refer to frogs and toads that tend to spend more of their life cycle in terrestrial habitats.

size:

The largest frog is the West African Gigantorana goliath. It is more than 30 cm in length from nose to anus, weighs 7.5 pounds - approx. size of a small baby; has been known to eat rats and even ducks
The smallest frog recorded is approx. 1 cm long (smaller than a dime); found in Cuba.
Largest American frog is the bullfrog (genus Rana) which reaches a body length of 20 cm.

Habitats and distribution:

Most abundant and successful of the frogs are of the genus Rana (Gr. for frog).
Found all over the temperate and tropical regions of the world except New Zealand, many islands, and southern regions of South America.

Usually found near water. Some, such as the wood frog spend most of their time in on damp forest floors and often some distance from water. It returns to water only to breed in early spring. Bullfrogs and green frogs are usually found in or near permanent water or swampy regions. The leopard frog R. pipiens found in a wide variety of habitats and with all its subspecies and different forms, is the most widespread of all North American frogs. Commonly used for lab studies. Has been found in nearly every American state and as far north as Canada to as far south as Panama. During winter, hibernates in soft mud of pond and stream bottoms.


Anatomical features of anurans

  1. External anatomy:
    1. Integument composition
      1. epidermis
      2. dermis
    2. Coloration of integument due to presence of chromatophores
Chromatophores
pigment cells of the dermis and epidermis; three types:
  1. xanthophores - located in uppermost portion of dermis and epidermis; contain yellow, orange, or red pigments
  2. iridiophores - located in middle layers of dermis; contain light reflecting (iridescent, "silvery") pigments
  3. melanophores - located in lower layers of dermis; contain brown or black colors due to varying amounts of melanin pigment (e.g. the black spots of the leopard frog we have been studying in lab (R. pipiens)
  1. Internal anatomy:
    1. Skeletal system:
      1. axial portion of endoskeleton
        1. skull
        2. vertebral column
        3. sternum (breast bone)
      2. appendicular portion of endoskeleton
        1. pectoral and pelvic girdles
        2. forelimbs and hindlimbs
    2. Circulatory system:
      1. Sinus venosus - portion of the systemic circuit which receives deoxygenated blood prior to flow into the heart
      2. Right atrium - chamber of the hear which receives deoxygenated blood due to contraction of the sinus venosus
      3. Left atrium - receives freshly oxygenated blood from the pulmonary circuit. Up to this point, deoxygenated blood and oxygenated blood are kept separated.
      4. Ventricle - receives both oxygenated and deoxygenated blood due to simultaneous contraction of the left and right atria; some partial mixing of the blood occurs, but not as much as would be expected.
      5. Spiral valve - located in ventricle and may act to help keep the blood from mixing.
      6. Conus arteriosus - chamber-like structure that directs the appropriate form of blood (oxygenated or deoxygenated) into systemic and pulmonary circuits, respectively.