Necturus (Mud puppy, Amphibia) possesses three pairs of external gills. Pila (Apple snail, Mollusca) respires by means of gills (ctenidium) in water and by a pulmonary sac on land. Millipedes (Arthropoda) respire by tracheae (tracheal respiration) and toads (Amphibia) exhibit buccopharyngeal respiration.

Land animals respire with lungs. With the help of lungs, these animals take in the oxygen in the air. However, fishes do not have lungs. They respire with gills. With the help of the gills, they breathe in the oxygen present in the water. Fishes take in water through mouth. The gill filaments absorbs oxygen from this water and distribute to the rest of body. 

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide. The gills are present in fishes cartilaginous fishes as well as bony fishes, in aquatic organisms such as prawns and mussels.
Therefore, the correct answer is option A.

Fish and some amphibians respire primarily with gills. Gills are projection of the skin. Gills are comprised of highly vascular structures, with their outer surfaces thrown into thin folds or narrow filaments to increase the surface area for the exchange of oxygen between the water and the blood. These gill structures develop entirely from the pharyngeal arch complex. Blood is supplied through the aortic arches that pass through each pharyngeal arch. The aortic arches branch to form the afferent branchial arteries and the capillary network in the gills. Oxygenated blood comes from the gills through efferent branchial arteries which supply the dorsal aorta, the vessel which is the primary supply of oxygenated blood to the body. Gills are not the projections of fins, alimentary canal and nostrils in fish. Thus, option A is correct.

A gill is a respiratory organ found in many aquatic organisms that extract dissolved oxygen from water and excretes carbon dioxide.complex or more active aquatic organisms usually require a gill or gills for respiration.  Gills has neither terrestrial nor pulmonary respiration because terrestrial respiration refers to respiration on land and pulmonary respiration refers to respiration facilitated by lungs. Hence, respiration by gills is neither terrestrial nor pulmonary.

The principle of counter current exchange can be illustrated using fish gills. In the gills, oxygen diffuses from an area of high concentration, the water, into an area of lower concentration, the fish’s bloodstream. First, consider what would happen under same or concurrent exchange. If oxygen-rich water and oxygen-deficient blood flow in the same direction, diffusion occurs into the bloodstream until equilibrium is reached. Thus, the maximum amount of oxygen that can enter the blood at any given time is only 50% of the total oxygen. On the other hand, if oxygen-rich water and oxygen-deficient blood flow in opposite directions, oxygen diffusion is continuous. This is because even though the oxygen concentration may decrease, the amount of oxygen in the water is always slightly greater than that in the blood, allowing diffusion to continue, and equilibrium to be avoided. In this case, the maximum amount of oxygen that can enter the blood is 90%, much greater than the amount which entered via concurrent exchange. 

A gill is a respiratory organ found in large aquatic organisms that extract dissolved oxygen from water and excretes carbon dioxide.
Tracheae is the cartilaginous tube descending from the larynx to the bronchi and carrying air to the lungs as in insects.
Malpighian tubules are involved in excretion (not respiration).
Book lungs is the respiration organ used for atmospheric gas exchange that is found in many arachnids.
Therefore, the correct answer is option C.

During the tadpole stage of the amphibian life cycle, most respire by means of autonomous external or internal gills. They do not usually have arms or legs until the transition to adulthood, and typically have a large, flattened tail with which they swim by lateral undulation, similar to most fish. As a tadpole matures, it most commonly metamorphosizes by gradually growing limbs (usually the back legs first, followed by the front legs) and then (most commonly in the case of frogs) outwardly absorbing its tail by apoptosis. Lungs develop around the time of leg development and tadpoles late in development will often be found near the surface of the water, where they breathe air. Tadpole does not respire through skin and air  bladder. 

Most fish exchange gases using gills on either side of the pharynx (throat). Gills are tissues which consist of cloth and fabric structures called as filaments. These filaments have many functions including the transfer of ions and water, as well as the exchange of oxygen, carbon dioxide, acids and ammonia. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In some fish, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like sharks and lampreys, possess multiple gill openings. However, bony fish have a single gill opening on each side. This opening is hidden beneath a protective bony cover called an operculum. Fish do not have lungs and do not absorb oxygen through mouth or tail. 

Although frogs have lungs, they perform much of their respiration through their skin, bypassing their lungs completely. Their moist skin is specially designed with tiny blood vessels near the surface to help diffuse oxygen in and carbon dioxide out, even when the frogs are hibernating or underwater. When a frog breathes through his skin, it's called as cutaneous gas exchange. This means he's exchanging oxygen for carbon dioxide, similar to the way your lungs exchange the gases. The gases pass through the skin by diffusion. Frog breathes through lungs when on land. Fish and earthworms can not breath through lungs as they dont have. 

Frog are amphibians and they can survive on land and water both. In water they breathe through gills while on land they have fully developed lungs to breathe. During hibernation, when they are not active they take oxygen from the moist skin by the process of diffusion. 

Gills are the respiratory organs of the fishes. It helps in gaseous exchange by absorbing the dissolved oxygen in water. The gills are covered by a bony covering which is known as operculum. It is used for respiration and feeding. Gills have gills lamellae through which the blood flows opposite direction to the water. The dissolved oxygen from the water diffuses into the blood capillaries.  

Frog are amphibians and they can survive on land and water both. In water they breathe through gills while on land they have fully developed lungs to breathe. It undergoes hibernation as it is cold blooded and the body temperature fluctuates with the surrounding. During hibernation the diffused oxygen is taken from moist skin.  

Fishes respire with the help of gills. Gills are richly supplied with blood capillaries and can readily absorb oxygen dissolved in water. But gills can not absorb gaseous oxygen. Since fishes cannot absorb gaseous oxygen they die soon after they are taken out of the water.

Gills are tissues which consist of cloth and fabric structures called filaments. These filaments have many functions including the transfer of ions and water, as well as the exchange of oxygen, carbon dioxide, acids and ammonia.

Frogs aren't restricted to breathing only through their lungs. Frogs that spend the coldest times of the year inside of the mud or within rotten heaps of leaves receive their necessary oxygen through their skin. This is referred to as cutaneous gas exchange. They use their skin to soak in the oxygen their bodies require. When frogs hibernate, they utilize the skin for any and all breathing. Damp skin is a must for subcutaneous gas exchange. If a frog's skin becomes dry, it will no longer be able to take in oxygen. Thus, option B is correct.

In animals, three main types of respiratory systems exist: respiration through the skin, respiration through the gills or respiration through internal air structures. Respiration through the skin, as in the hydra, involves the passing of molecules into and out of the body via diffusion. Aquatic animals use gills, which are structures composed of tissues and blood vessels that extend from the body, for their respiration needs. Most terrestrial animals use internal air structures, such as lungs or tracheae, to respire. 

While both the tadpole and the frog like other organisms need to take in oxygen from the environment, the respiratory mechanisms differ. The tadpoles make use of gills and the mature frogs respire need to take in oxygen from their environment, and the way they respire also changes as they develop. Tadpoles use gills, while mature frogs have three types of respiration- skin, buccal pumping and lungs

In animals, three main types of respiratory systems exist: respiration through the skin, respiration through the gills or respiration through internal air structures. Respiration through the skin, as in the Hydra, involves the passing of molecules into and out of the body via diffusion. Aquatic animals use gills, which are structures composed of tissues and blood vessels that extend from the body, for their respiration needs. Most terrestrial animals use internal air structures, such as lungs or tracheae, to respire. Cockroach, snail and earthworm has blood. 

Animal P always breathes through gills - fish; animal Q breathes both through moist skin and lungs - frog; animal R breathes only through its moist skin - earthworm; animal S breathes through its cell membrane - amoeba. Oxygen diffuses in and carbon dioxide diffuses out of its cell membrane.

In frogs the process of respiration takes place through three respiratory surfaces - skin, lungs and lining of the mouth. However, when submerged in water, the frog respires through its cutaneous membranes on the skin as it is quite permeable to water and contains a large network of blood vessels. The cutaneous membrane allows the respiratory gases to readily diffuse directly. Even when out of water, mucus glands present in the skin help in the cutaneous respiration.

Respiration of aquatic animals is a process wherein they obtain the oxygen from the water. The large aquatic animals have well developed gills which helps them in their respiratory function. 

Double trachea is a type of the respiratory tract found in penguins, spoonbills, petrels, dugongs and sea lions shows a high bifurcation of the trachea with fused primary bronchi and intra tracheal septum.

Earthworms have no special respiratory organs. Gases are exchanged through the moist skin and capillaries, where the oxygen is picked up by the hemoglobin dissolved in the blood plasma and carbon dioxide is released.
The basic respiratory system of insects consists of a series of rigid tubes, called as tracheae.
In most of the fishes, the respiration takes place through gills.
The birds have lungs, air sacs and a syrinx and their respiration requires two cycles to move a volume of air.
Therefore, the correct answer is option A.

Respiration in frogs is both through lungs and skin. However, during hibernation, frogs make use of the skin to breathe. However, there are exceptions such as the wood frog which stops to breathe completely during its hibernating period. During hibernation most of these amphibians receive their necessary oxygen through their skin referred to as cutaneous gas exchange. Their skins soaks in the oxygen required by their bodies require.

Freshwater fish lives in rivers and lakes where the salinity levels are less than 0.05%. The freshwater species differ physiologically from salt water fish in that their gills are modified to diffuse dissolved gasses while retaining the salts present in the body fluids. Further, their scales reduce water diffusion through the skin and also have well developed kidneys that can reclaim salts from body fluids before excretion. Through this process, they get rid of water and acquire salt.

The mucus glands present in the skin of the frog helps to absorb dissolved oxygen from the air. However, the frogs have three respiratory surfaces and exchange of gases with the surroundings through the skin is a continuous process.
Therefore, the correct answer is option D.

Water naturally seeks a chemical balance, or equilibrium. That means water flows from areas of higher water concentration to areas of lower water concentration to equalize the system. Water concentration inside a fish is higher than in the ocean itself because the ocean is so salty. As a result, most saltwater fish constantly lose water through their gills and skin. Because the fish is losing water, it must drink a lot to stay hydrated-but salty seawater is the only water around. To get rid of excess salt, the fish's kidneys pump lots of salt into its urine.

Gills are the respiratory organs of large aquatic animals. These organs are specifically adapted to their function. In Osteichthyes, the gills have four gill arches, Chondrichthyes have two, fish has 7 gill baskets with a long bony cover for the gills which is used to push water. 

Hydra doesn't have blood. It is a single-cellular animal. In the hydra, respiration is accomplished via diffusion of molecules of oxygen and carbon dioxide through the skin whereas earthworm, cockroach and octopus has blood. Thus, option B is correct.