Anatomy of a Frog - Internal and External

Frogs and toads are from the order Anura. While we rightly associate tadpoles with anurans, not all frog species have a tadpole stage^[1]. They all have a larval stage and require water to develop. Frogs that don't have a tadpole stage go through direct development. This means they emerge from the egg as a froglet, i.e. a miniature version of their adult frog stage. This occurs in a minority of frog species as most larval stages are tadpoles.

Tadpoles have a tail which allows them to swim in water, an appendage which is not carried over into their adult stage. Tadpole shave other anatomical structures which allow them to survive in water including gills. They lose these structures due to the process of metamorphosis. Animals that go through metamorphosis change significantly from larval to adult stages.

We now look at some of the most important internal and external anatomical features of tadpoles:

1. Head: as shown in the diagram below, you can see the head is significantly differentiated from the body. Although they lack eyelids, tadpoles have eyes which are located towards the front of the head.
   
   
2. Mouth: they do not have true teeth, but tadpoles have similar structures made of keratin that are located in the upper jaw used for feeding. They may also have these structures in the lower jaw surrounded by a kind of horny beak, but this can vary between frog species. Larvae usually follow a herbivorous diet, but in certain circumstances they can be omnivorous, even eating other tadpoles. Learn more with our article on what tadpoles eat.
   
   
3. Body: it is elongated. Tadpoles are exclusively aquatic in habit. For this reason, their head is differentiated head from an elongated body which ends in a tail. If they resemble fish it is precisely because this body shape allows them to swim in water. They do not have other limbs since they do not need them until after they undergo metamorphosis.
   
   
4. Tail: a hardened muscular structure that covers the notochord, a cartilage-like structure which is important for sending signals during development. It is an anatomical structure common to all chordates, giving rise to their nomenclature. The tissue of the tail is rigid, but it must be able to decompose during metamorphosis. This occurs due to apoptosis, a type of programed cell death. Tadpoles are able to shed their tails in some circumstances, such as when attacked by predators.
   
   
5. Belly: tadpoles have a bulging belly which is part of their digestive system. This includes a digestive tube which starts at the mouth and connects to the intestine. Another tube located towards the end of the tail known as the ventral tube is used for excretion.
   
   
6. Skeleton: is of a cartilaginous type, but these structures will also transform until bones are formed in adult individuals. As usually occurs in vertebrates, the notochord is transitory (as we have commented) and gives rise to the vertebral column, also ossified into bone in frogs.
   
   
7. Respiratory system: they have external gills for breathing. As the tadpole develops, internal gills appear below the initial external ones and are covered by a fold of tissue. Once formed, they create tiny openings to the outside known as spiracles. The tadpole uses the internal gills for gas exchange, but the adult frog does not. Similarly, tadpoles have two nostrils that will later be the nasal openings in the adult.

As summation, the anatomical structures of a tadpole are as follows:

• Head
• Eyes
• Nostrils
• Oral disk or mouth
• Keratin dental structures
• Spiracle
• Digestive tube
• Cartilaginous skeleton
• Tail
• Cloaca

Once metamorphosis occurs, some tadpole structures are transformed or lost when they become adult. The reason for this is mainly due to their habits, since adult amphibians can live on both land and water. For a frog to have both aquatic and terrestrial habits, they require various internal parts of their anatomy. We explain what they are, as well as show their location in the diagram below.

Musculoskeletal system

Frogs are vertebrate animals, so once fully developed they have muscles and a bone structure. Despite their size, this structure is complex. They have around 30 bones which are also present in other vertebrates. However, several of a frog's bones and muscles have been modified to allow them their supreme jumping ability. This is because they move in a saltatory way, leaping from one spot to another rather than moving gradually.

Depending on the species, there may be variations in musculoskeletal structure. For example, some frog species are arboreal and have forelimbs which are adapted to provide a specific dexterity and grasping ability^[2]. Learn more about the musculoskeletal system of these animals in our related article on whether frogs have bones.

Nervous system

The nervous system of frogs consists of the following main structures:

• Brain
• Spinal cord
• Cranial and spinal nerves

The frog's brain has olfactory lobes, two hemispheres, the pineal body, the optic lobes and the cerebellum, as well as the medulla oblongata. They are responsible for controlling the various senses of the frog which make them both able to predate on other animals, as well as avoid being prey themselves.

Respiratory system

The respiratory system of frogs includes lungs, organs present in adult frogs, but not in their larval stages. In this way, we can say that frogs have pulmonary respiration. Frogs use their mouth to pump air through their respiratory system into their lungs.

Frogs are also animals that can breathe through their skin. This is because their skin is semi-permeable, something especially useful for frogs living in mainly aquatic habitats. This is because this habitats can be deficient in oxygen.

Circulatory system

Like other amphibians, frogs have a heart with three chambers through which blood passes to be oxygenated. It does do so by entering through separate atria. These chambers contract to move blood to the ventricle, then pass through a valve and reach the aorta if it is oxygenated blood or the pulmonary artery if it is deoxygenated.

Digestive system

The frog's digestive system begins in the mouth. Frogs have some teeth that are used to hold prey, but not for chewing. They also have a tongue which is especially adapted to catch prey. It is extendable and has a process of adhesion which is not well understood, but it is believed to be due to a distinct tongue musculature and specially adapted mucus fibrils^[3].

Once captured, the food moves through the esophagus to the stomach. It is then mixed with digestive enzymes to pass into the small intestine and continue digestion. Within this system, frogs also have a pancreas, large intestine, gallbladder, spleen and its own vascular system. Learn more about frog diet with our article on what do frogs eat?

Excretory system

The parts of the excretory system of frogs are made up of the kidneys, ureters, bladder and cloaca. All these organs are involved in the storage, transport and expulsion from the body of all those waste substances produced by metabolic processes. The cloaca is the orifice for the reproductive, digestive and urinary systems.

Reproductive system

Frogs do not have all the reproductive organs common to many other vertebrates. In the case of males, they have two testicles associated with the kidneys, efferent ducts and urogenital ducts. Females have ovaries also linked to the kidneys and oviducts.

Learn how these processes are carried out with our article on frog reproduction.

Visual and auditory system

Frogs have good binocular vision which is why they have internal nerves associated with the visual system. Although they don't have ears, they do have eardrums, as well as middle and inner ears. This means frogs are able to hear. Although we do not know the extent of their auditory ability, there is some evidence to suggest certain frog species have ultrasonic communication abilities^[4].