Salamanders range from 4 cm to 1.5 m in length, have glandular skin and lack claws. Most have four legs except sirens, which lack pelvic limbs.
Caecilians range from 7.5 cm to 75 cm in length and lack limbs. A short tail is present in some species. A unique feature is the presence of small olfactory and sensory tentacles in the nasolabial groove.
Skeleton
| Order | Features |
|---|---|
| Anurans | 4 legs with 4 toes on the front and 5 toes on the back |
| Fused vertebrae—presacral, sacral, postsacral | |
| Lack sacrum | |
| Pelvic girdle is fused | |
| Forelimb—scapula, humerus, radioulnar joint, carpals, metacarpals, phalanges | |
| Hind limb—femur, tibiofibular joints, tarsals, metatarsal, phalanges | |
| Urostyle—fused caudal vertebrae | |
| Caudata | 4 legs but hind reduced in mud eels and missing in sirens |
| 4 toes on the front and 5 toes on the back | |
| Capable of limb and toe regeneration | |
| Tail autotomy | |
| Caecilians | Lack pectoral/pelvic girdle and sacrum |
CARDIOVASCULAR
Amphibians have a three‐chambered heart with two atria and one ventricle. There is an interatrial septum that is complete in anurans but fenestrated in caecilians and most salamanders. This septum may allow for some mixture of oxygenated and deoxygenated blood. A renal portal system is also present. A unique adaptation of amphibians is the presence of an extensive lymphatic system with lymph hearts, lymph sac dilations surrounded by muscle, which allows them to beat independently of the heart. The lymph hearts provide unidirectional flow of lymph to the heart and can beat at a rate of up to 60 beats/minute. The lymph contains all blood components except red cells.
VENIPUNCTURE SITES
Venipuncture can be challenging in many species but there are various sites where blood can be accessed. Sedation or anesthesia may be required to successfully collect blood, especially if attempting cardiac puncture. Venipuncture sites include the femoral vein, ventral abdominal vein, lingual vein/plexus, tail vein, maxillary/facial vein, musculocutaneous vein, and the heart.
IMMUNE SYSTEM
The blood of amphibians is composed of erythrocytes, thrombocytes, monocytes, lymphocytes, eosinophils, basophils, and neutrophils. Morphology of the white blood cells can be quite variable, so obtaining accurate white cell counts can be challenging when working with laboratories not familiar with their cell morphology. Their function is presumed to be similar to other species, but further research is still needed in this area.
Some amphibians lack a truly functional bone marrow, so hematopoiesis can occur in the spleen, liver, and kidneys, according to age and species. The spleen is composed of a red pulp responsible for erythropoiesis and white pulp responsible for myelopoiesis. The thymus produces T‐lymphocytes and is functional through life. The size of the spleen and thymus are affected by seasons, nutritional status, and stress. A well‐developed gut associated lymphoid tissue is present instead of lymph nodes.
RESPIRATORY SYSTEM
Amphibians have unique adaptations for respiration. In addition to traditional lung (pulmonic) respiration, they can also exchange gas via buccopharyngeal respiration, which allows exchange at the level of the oral and pharyngeal cavity. Cutaneous respiration may comprise up to 80% of the gas exchange, especially in aquatic species. Lateral folds, costal grooves, or cutaneous hairs may be present to increase surface area of the skin and improve the efficiency of cutaneous respiration. Finally, branchial/gill respiration occurs in juvenile stages and in neotenic species.
Amphibians have short tracheas with complete rings and single chambered lungs without alveoli. Instead there are a series of infoldings of the lungs that increase surface area. Lungs are absent in Plethodontidae and reduced or absent in the Hynobiidae.
Anuran larvae have small, simple gills covered by an operculum similar to fish. In contrast, salamander larvae have external gills without an operculum. The gills of caecilians are resorbed before birth/hatch. In anurans, they are resorbed during stages of metamorphosis. Salamanders resorb their gills during metamorphosis but they are retained in neotenic species.
INTESTINAL TRACT
The majority of adult amphibians are carnivores, but larvae may be herbivores. Sirens may also remain omnivorous into adulthood. Caecilians rely on olfaction to find prey while anurans and urodeles rely primarily on sight. Amphibians have jointed pediceled teeth in which the crown is loosely attached to the base (pedicel) of the tooth. The tooth itself attaches to the jaw. Teeth are used for holding prey in place rather than chewing, so they are often angled backward toward the pharynx. Teeth are shed and replaced through life. There are one or two rows of maxillary and mandibular teeth in most species. However, ranid frogs lack mandibular teeth and bufonid toads have no teeth. Anurans and salamanders have well‐developed moveable tongues commonly used for prey apprehension. Caecilians have fixed tongues. Frogs in the family Pipidae (clawed frogs and Surinam toads) lack tongues and vocal cords.
LIVER
Liver anatomy varies among amphibian species. Anurans have a more traditional bilobed liver while in Caudates and Caecilians it is elongated and marginated. The gall bladder is usually well developed and intimately associated with the liver. A bile duct connects to the duodenum or pancreatic duct in some species. The liver is a significant erythropoietic center and also contributes to leukocyte production during metamorphosis. A large amount of pigmented melanomacrophages and non‐pigmented Kupffer cells can be found and contribute to immune function. The amount of these cells present at any given time is influenced by seasonality. They also increase in numbers with age and antigenic stimulation. Melanomacrophages are commonly identified in aspirates of animals with coelomitis.
URINARY SYSTEM
Amphibians have mesonephric kidneys that cannot concentrated urine above plasma osmolality. A single or bilobed urinary bladder is present in most species. Larvae and aquatic species are primarily ammoniotelic while terrestrial species are primarily ureotelic. There is a lesser number of primary uricotelic species (waxy tree frogs). Clawed frogs have the ability to be ammoniotelic or ureotelic depending on water availability.
REPRODUCTION
Amphibians have paired gonads consisting of either testes – Wolffian duct – cloaca, or ovaries – coelom – infundibulum – oviduct.
The Bidder’s organ is a remnant of ovarian tissue in male bufonid frogs and does not indicate hermaphroditism. Salamanders have internal fertilization but lack an intromittent organ. Instead,