Lung Fishes as an Intermediate Form Between Fishes and Amphibians : Anatomy, Distribution and Origin

Lung fishes are so named because their swim bladder is developed into a well formed lung like structure. They are also called Dipnoan because two internal nostrils are present. According to several scientists they are an intermediate form between Fish and Amphibia and they are closely related to Tetrapods. They show several common features between Fishes and amphibians. The surviving genera of Lung fishes are Protopterus , Neoceratodus, and Lepidosiren. They are mainly inhabitants of the rivers (though Protopterus species lives in Lake Ecosystem) and they breathe in air. Neoceratodus is found only in Burnett and Mary rivers in Queensland of Australia, Lepidosiren in the rivers of tropical South America and Protopterus in the tropical Africa and they can stay after the complete dry up of the rivers. They dig to insert themselves into the mud leaving a small opening for breathing and can remain at this condition for at least 5-6 months. Dipnoans probably originated during Devonian period.

Salient features of Lung Fishes :

a) Ectodermal scales are rudiment to form bony plates.

b) Tail is diphycercal (symmetrical).

c) Vertebrae of vertebral column are cartilaginous.

d) Swim bladder is developed into lung like structure divided into several chambers (In Neoceratodus only one lung is present but in other species two lungs are present. The wall of the modified air bladder (swim bladder) contains muscles and elastic fibres and the cavity is subdivided into a number of pouches or alveoli.

e) Separate dorsal fins are absent.

f) External nare present at the edge of the mouth and the internal nares open into it's roof.

g) In Protopterus and Lepidosiren there is an epiglottis and second and third gill arches bear no lamellae.

h) Pulmonary artery arises from dorsal aorta.

i) Pituitary gland (Hypophysis) shows several common structural features between Teleostian and amphibian animals.

j) The anatomical peculiarities and the physiological features of nervous system show the similarity with the Amphibians.

k) The urino-genital system is quite similar with the amphibians.

l) The developmental biology is quite similar with that of Amphibians.

m) Inferior venacavae are present.

n) Stomach is absent and the intestine is ciliated.

o) No hepatic caeca but a well formed spiral valve is present.

p) Lungs open into the oesophagus ventrally.

Discussion :

Lung fishes show several features, some of them are similar with fishes and some of them are similar with amphibians. They have notochord as an unconstricted rod, diphycercal tail, swim bladder, large red blood cells etc. those are quite similar features with fishes but the modification of their swim bladder into lung like organization, nervous system, developmental patterns, urino-genital system etc. are also similar with Amphibians. Besides their pituitary, respiratory system etc. shows their similarity with both Fishes and Amphibians. Therefore these features are enough to put them between the Fishes and Amphibians during the evolutionary mode of approach.