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Air breathing evolved long before fish moved on to land. When water oxygen levels are low, many fish supplement their oxygen intake using lungs or other respiratory structures (facultative air breathers).  Anabantid fishes (eg Siamese fighters), for example, have a labyrinth organ that extracts oxygen from gulped air. But some fish are obligative air breathers such as electric eels and snakeheads, they drown if they can’t get to the surface to breath.  

Lungs develop as out-pockets in the upper digestive tract during embryonic development. The lungs of bichirs (African air breathing fish), lungfish and tetrapods originate from the ventral surface, whereas those in garfish and teleosts develop dorsally. Lungs function by increasing the surface area to maximise oxygen uptake. The alveolar lung increases surface area by having little pockets as seen in garfish, lungfish and some tetrapods. Surface area can also be increased by adding a second lobe as seen in some lungfish and tetrapods. Bichirs have non-alveolar lungs with two lobes but one is smaller than the other.

For gaseous exchange

For buoyancy in water

To survive longer in oxygen diminished environments 

See the ref on iLearn: Maina JN (2002) Structure, function and evolution of the gas exchangers: comparative perspectives. Journal of Anatomy, 281-304.  

Sarcopterygian – Lobed Finned Fishes:

Australian Lungfish: Observation Of Preserved Specimens

  • Subphylum: Vertebrata
  • Class: Osteichthyes  
  • Subclass: Sarcopterygii
  • Order: Dipnoi
  • Family: Ceratodidae
  • Species: Neoceratodus forsteri(Australian lungfish)

Lungfish provide an interesting insight into the evolutionary forces that motivated the movement on to land. But most of these early changes actually took place in shallow water. They were once widespread in Gondwana but most species are now extinct. Australia has just one species left. The lung from the lungfish is closely related to the swim bladder in fish. Some of the lungfish evolved a connection between the nares and the mouth, so they could breath through their nose. The Australian species is a facultative air breather, only using its lung when water oxygen levels are low. Because of the twin sources of oxygen (lung and gills) a more elaborate circulatory system is required. One loop serves the lung and the other the gills. The heart too, is a little more complicated as a consequence with twin atria. Of course, they are most relevant to vertebrate evolution because their limbs and associated girdles are the origins of tetrapod limbs.

Examine the preserved lungfish. Identify and label the external features of juvenile lungfish on the diagram below.

  • Paired fins               • Cloaca
    • Operculum           • Eyes
  • External nasal openings (leading to internal nares)
    • Lateral line             • Electroreceptors
    • Scales                     • Tooth plates
  • Caudal fin 

Take the time to examine the teeth – in the upper jaw, there are two rows of tooth plates attached to the palate (no marginal jaw bones and teeth) – in the lower jaw there are two matching tooth plates which work against the upper two to crush food items drawn into the mouth.

Describe the difference in the position and appearance of the fins of the Actinopterygian and Sarcopterygian?

Sarcopterygian has two dorsal fins that has separate bases while Actinopterygian has only one dorsal fin

Why is this comparison important when considering the origins of tetrapods?

This is because the braincase of Sarcopoterygians had hing line but has been lost in tetrapods and lungfish. 

Provide three hypotheses explaining the selective pressures that might lead to fish acquiring lungs

Image and label a dissected lungfish with the following labels listed below

(see notes on iLearn to help)

  1. i) Digestive system
    Locate the following structures:
  • Liver • Gall bladder
    • Oesophagus – hard to see much of it at this stage
    • Intestine • Cloaca
  • Pancreas and spleen (located within the spirals of the duodenum)

What organ is absent from digestive system and what does the lungfish have in its absence? 

Lung fish has no stomach but has an intestinal spiral valve 

Locate the following structures:

  • Lung
  • Gonads (immature as juvenile). The gonads run along either side of the lung. Testes tend to be whiter than the yellowish ovaries
  • Kidneys, positioned under mesentery.
  • Trachea (single) leads off from the oesophagus to the lung. Note the extent to which the lung fills the body cavity.

What organ in the Actinopterygian is said to be homologousto the lung in Sarcopterygii? 

The swim bladder is homologous to the lungs of Sarcopterygii

How do the two organs differ in structure and function?

Swim bladder is sac that contains gas and this allows the fish to alter its buoyancy and its position in water by adjusting the volume of gas in the sac. Lungs is has an interconnection f blood vessels that allows gaseous exchange to occur freely.

How many lungs does the Australian lungfish have?

It has one lung only 

The diagram below illustrates blood flow through the circulatory systems of fish and lungfish.

How doesthe structure of the heart of lungfish differ to that of the fish?

Lungfish has double circulation while the fish has one circulation

Lungfish has four chambers while fish has two chambers

Lungfish has gill bypasses while fish has no gill bypass

Lungfish have a lung while fish has no lung but breaths through gills

What does it infer are the key differences between the circulatory system of the fish and that of the lungfish?

Lungfish has a better oxygenated blood supply compaired to that of fish

  • Subphylum: Vertebrata
  • Class: Amphibia  
  • Order: Anura
  • Family: Bufonidae
  • Species: Rhinella marina (Cane toad)

Amphibians are interesting beasties with split lives. Most have aquatic larval stages (tadpoles) whose anatomy is pretty much the same as a fish. But they undergo an amazing transformation when they become terrestrial adults (metamorphosis). The adult circulatory system is much like the lungfish except rather than one branch going to the gills, it goes to the skin, which provides an important respiratory surface. Unlike other vertebrates, frogs don’t have ribs (secondarily lost) and they rely on buccal pumping to inflate their simple lung. When breathing with their lung, the air is taken in through the nostrils. Their skin works in combination with their kidneys to control osmotic balance. Kidney function is similar to freshwater fish. The skeleton is highly modified for hopping. Eggs are laid in the water because they lack the amniotic membrane seen in the rest of the terrestrial tetrapods (amniotes). Adult frogs have an operculum-columella complex (2 bones) that transmit vibrations to the inner ear to enable them to hear in air. 

Important Note:

Cane toads secrete nasty toxins.

For Your Safety

1) you are required to wear safety goggles, gloves and a lab coat when dissecting the toads – you will not be allowed to dissect the toad without these three items; 

2) do not touch your bare skin while wearing gloves that have been in contact with the specimen; 

3) wash your hands carefully before leaving the laboratory 

  • Try to determine the sex of your toad.The following may give away the sex:
    • Nuptial pads – males during breeding time have keratinised bumps on their first two fingers.
    • Males often have a warty spine while the females have a smooth spine.
    • Females are significantly larger than the males.
    • External nares, Tympani,
    • Bony ridge, Parotoid glands,
    • Nictitating membranes. 

What are nictitating membranes for?

They serve to protect the globe, generate immmunoglobulins, part of precorneal tear film and spread tears.

Does the toad have webbed feet?

The toad has no webbed feet

  • Cut into one of the parotoid glands – WARNING you and your colleagues must have gloves, safety goggles and a lab coat before doing this step. 

The toxin from this gland is incredibly toxic. It is responsible for the deaths of thousands of animals that attempt to eat cane toads. Cane toads are not the only amphibians in Australia that are highly toxic. Yet death adders consume these toads. Propose an idea on how death adders may achieve this?  

Death adders attract toads and kill them the waits for the poisonous toxins from the toads parotid gland to degrade before consuming it.

  • Turn the frog on its back and pin down the legs. Cut the hinges of the mouth and open it wide. Locate and identify the following structures inside the mouth. Use a probe to help find each part: the internal choana (nostrils), the tongue, the openings to the Eustachian tubes, the oesophagus, the pharynx and the slit like glottis.

What is so special about the internal choanae (nasal passage)? 

They allow breathing when mouth is closed in toads that have secondary palates

All extant lungfish have internal nostrils which are suggested to be homologous to choanea (although this is debated) except for the Australian lungfish can you think of a reason why?  

This is because Australian lungfish has one pair of lung and often has to surface to take in oxygen so their nostrils are externally placed

Does your toad have teeth?

Toads do not have teeth

Based on your answer how do you think these animals eat?

They use their sticky tongues to capture preys and swallow them whole

  • Pin the toad to the corkboard through the feet and hands so the arms and legs are fully extended.
  • Cut 1 – Using the forceps lift the skin and use scissors to cut along the centre of the body from the cloaca to midway between forearms. Making sure you try and keep the scissors parallel to the surface of the toad.
  • Cut 2 + 3 – Cut toward the side at each leg as depicted in the figure below. Now pin the skin out flat.
  • Cut 4 + 5 – Cut to the lip of the frog. Then make a cut following the curvature of the jawline. Now pin the skin out flat. 

  • Repeat the same steps to cut through the membrane protecting the visceral mass. Lift and cut through the muscles and breastbone to open up the body cavity. If your frog is a female, the abdominal cavity may be filled with dark-coloured eggs. If so, remove the eggs on one side so you can see the organs underlying them. 
  1. Circulatory and digestive systems

Label the image with the following:

  • Lungs Oesophagus
  • Stomach Small intestine
  • Large intestine Rectum  
  • Liver Gall bladder
  • Pancreas Thyroid
  • Spleen Heart 


Is the lung connected on the ventral or dorsal side of the oesophagus? – To answer this question you will need to remove the liver.

On the ventral side of esophagus

How does this compare with the connection of lung in Sarcopterygii and the swim bladder in Actinopterygian?

They differ in position such that the swim bladder is located on the dorsal side relative to oesophagus in Actinopterygii to control buoyancy

Why is this distinction important, from:

an evolutionary standpoint– (Hint: think about where your lungs are)

It helps in locating the lungs anatomically and its efficacy in gaseous exchange

a functional perspective –(Hint: what role do these organs serve besides from respiration – note important for mobility of aquatic animals) 

Swim bladder maintain buoyancy while the lung serve gaseous exhange 

  • Use a probe (or use forceps as a probe) and scissors to lift and remove the intestines. To do this cut through the top of the oesophagus and the base of the rectum. You may need to peel away mesentery tissue just use two forceps to peel away the tissue.
  • Once the intestines have been removed use the diagram below to identify the parts of the urinary and reproductive systems. Find and label the following features;
    • Bladder, Cloaca, Eggs, Fat bodies, Kidney, Oviducts, Testes, Ureter, Uterus. 

A Ovary

B  Oviduct

C Testis

D Kidneys

E Cloaca

F Bladder

G Ureter

H  Rectum

I Fat body

  • Remove the kidneys. You should now be able to see the spine of the toad. Examine the relationship between the structure of the urostyle and leg movement. Grab the spine with one set of forceps. Then using another pair grab one of the vertical bones of the urostyle and move it back and forwards. You should see the legs moving in unison.
  • Along the spine you should see the threadlike sciatic nerves, which are white, cord-like and run along the spine extending into the limbs. Attempt to follow the nerve into the leg by dissecting the thigh. Note the size and texture of the leg muscles. Can you see any other nerves in the body?

Before finishing see if you can locate the brain.

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