DIGESTIVE SYSTEM
Although many larval amphibians are herbivorous, adults are entirely carnivorous, with a wide variety of invertebrates constituting a large part of the diet. Caecilians rely primarily on olfactory cues to locate prey, whereas salamanders and anurans use sight as the prominent sense for food detection
Amphibians
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(Stebbins & Cohen 1995).
Prey movement triggers the feeding response. Anurans in particular are voracious feeders and tend to eat anything that fits in their mouth. Gastric overload and impaction, as well as ingestion of non-food items, such as substrate gravel or moss, are fairly common (Fig. 1.10).Dentition
All orders of amphibians have “jointed” pediceled teeth. The crown is loosely attached to the base, or pedicel, of the tooth that is in turn attached to the jaw. Crowns are typically recurved in the direction of the pharynx and function in holding prey as opposed to chewing. The teeth are shed and replaced throughout life. Caecilians, salamanders, and some anurans have one or two rows of maxillary and mandibular teeth. Ranid frogs lack mandibular teeth, and bufid toads do not have any teeth. Many species also have vomerine and palatine tooth patches on the roof of the mouth (Stebbins & Cohen 1995; Wright 2001c).
Tongue
The tongue of most anurans and salamanders (caecilians have fixed tongues and pipid frogs are tongueless) can be extended beyond the mouth for food capture (Stebbins & Cohen 1995; Wright 2001c). In some species the tongue may be projected up to 80% of the total length of the animal (Mitchell et al. 1988). The tongue is extended and flipped (such that the posterodorsal aspect of the folded tongue becomes the anteroventral aspect), the surface of the tongue adheres to the prey item and is subsequently retracted into the mouth (Stebbins & Cohen 1995). The entire process may take as little as 50 milliseconds (Mitchell et al.
1988). Once in the mouth, the floor of the mouth is raised and the eyelids are closed, forcing the globes ventrally. This pushes the food item caudally into the pharynx.Liver and intestinal tract
The remainder of the intestinal tract is relatively short and follows the normal vertebrate plan. Feces are expelled into the cloaca, a common opening for the gastrointestinal, urinary, and reproductive systems.
The amphibian liver is located posterior and ventral to the heart. The gross anatomy is variable depending on the taxonomic group but generally conforms to the body shape of the amphibian. Anurans have a bilobate liver, while cau- dates have a slightly elongated and marginated liver, and in the caecilians it is slightly marginated and very elongated. The gall bladder of all the groups is intimately associated with the liver, with a bile duct connecting it to the duodenum. In some species it joins the pancreatic duct before it enters the intestinal tract (Duellman & Trueb 1986).
From early embryonic stages through to the adult stage, the liver serves as an important erythropoietic center in amphibians. In addition, through the metamorphic stages, there is an increase in hepatic leukocyte production (Chen & Turpen 1995), and the liver plays an important role in immune function with its relatively large population of pigmented melanomacrophages and non-pigmented Kupffer cells (Gallone et al. 2002; Guida et al. 1998). The numbers of hepatic melanomacrophages in the amphibian liver are influenced by seasonal variation in some species, and increase with age and with antigenic stimulation in all species (Barni et al. 1999; Sichel et al. 2002; Zuasti et al. 1998). It is not uncommon to find melanomacrophages on celomic aspirates in amphibians with celomitis or ascites.
As with higher vertebrates, the amphibian liver also plays an important role in the synthesis of nitrogenous compounds, anti-oxidation reactions, metabolism of various endogenous and exogenous substances, glucose metabolism, protein synthesis, lipid metabolism, and iron metabolism (Crawshaw & Weinkle 2000).