Explore topic-wise InterviewSolutions in .

From Hydrostatics it is known that an object does not sink if its density is equal or less than the density of the fluid in which it is immersed. 

When the swim bladder is filled by gas it reduces the density of the fish body and when it is emptied this density is increased. So this mechanism controls the fish depth under water.

In chondrichthyes fecundation is internal by means of copulation. In osteichthyes fecundation generally is external and the gametes are released in the water, where they can fecundate their counterpart and form the zygote.

Compared to cyclostomes, evolutionary novelties presented by fishes are: pectoral and pelvic fins, symmetric and paired; the presence of mandibles.

In amphibians the embryonic development is indirect (there is a larval stage).

The amphibians are totally aquatic in the larval stage and partially terrestrial animals as adults and for these facts they are considered intermediate beings in the evolutionary passage of vertebrates from the aquatic to the dry land habitat. Amphibians are also the first tetrapod animals, i.e., the first with two pair of limbs, a typical feature of terrestrial vertebrates. The name “amphibian” comes from the double life (aquatic as larvae and partially terrestrial as adults) of these animals.

“Chondros” means cartilage, “ictis” means fish (both from the Greek); the name chondrichtians is for fishes with cartilaginous endoskeleton. The name osteichthyes comes from the existence of a bony endoskeleton in these fishes (“osteo” means bone, from the Greek too). 

Fishes are all aquatic animals and thus they have a hydrodynamic elongated body suitable to move under water, without limbs and with fins. The habitat conditions the branchial respiration too.

The larva of amphibians has exclusively branchial respiration. This is one of the reasons why it depends on water to survive.

Adult amphibians have kidneys that filter blood. Nitrogen waste is excreted as urea (so amphibians are ureotelic beings). The larvae, aquatic, excrete ammonia.

Examples of representing beings: frogs, toads, salamanders. Basic morphology: two pairs of limbs, eyelids, hydrodynamic larvae. Skin: moist and permeable, mucous glands. Respiration: cutaneous and pulmonary, branchial in larval stage. Circulation: closed, incomplete, heart with three chambers without interventricular septum. Nitrogen waste: urea. Thermal control: heterothermic. Types of reproduction: sexual, water dependant, external fecundation and aquatic larval stage.

Permeable skin, body subject to dehydration, external fecundation, eggs without shells and larval stage with branchial respiration are features that make amphibians dependent on water to survive.

Fishes have a pair of kidneys that filtrate the blood. Bony fishes excrete nitrogen as ammonia, NH3, (they are ammoniotelic) and cartilaginous fishes excrete urea as nitrogen waste (they are ureotelic, like adult amphibians and mammals).

The fish heart has only two chambers, an atrium and a ventricle, and the blood that comes to it is purely venous.

In amphibians there are three heart chambers (a second atrium is present) and there is arterial blood coming from the lungs; in these animals the heart has two atria (one that gets blood from the body and other that gets blood from the lungs) and one ventricle; arterial blood mixes with venous blood within the ventricle which in turn pumps the blood to the lungs and to the systemic circulation.

As in every vertebrate the circulatory system of fishes is closed, i.e., blood flows only within blood vessels.

The fish heart has only two consecutive chambers: a thin-walled atrium and a muscular ventricle. The arterial (oxygenated) blood comes from the gills and gains arteries towards tissues, then venous blood is collected by veins and reaches the atrium of the heart passing to the ventricle that pumps the venous blood towards the gills to be again oxygenated.