Scoliodon (fish)is a poikilothermic cartilaginous fish. It is commonly known as shark. Rana(frog) is an amphibian and poikilothermic animal. It is commonly called as frog.

Living cells require energy for performing metabolic activities. This energy is provided by the oxidation of food materials present within the cells. In oxidation, Oxygen is consumed and produces carbon dioxide and water. Carbon dioxide is harmful to the protoplasm. Hence to keep the oxidation continuously, there is a constant need for the supply of oxygen to the cells and removal of carbon dioxide from them. Oxygen is obtained from the environment (water or air) and in return carbon dioxide is added to the environment. The exchange of oxygen of the environment with the carbon dioxide of the body is known as 'respiration'. In all vertebrates, some organs of. The body are specialised for the exchange of gases. These are called respiratory or breathing organs. The surface of these organs at which the exchange takes place is called as 'respiratory surface. 

Frog breathing-respiration

Vertebrate respiratory organs include gills and lungs. Both of these organs develop from the pharynx. Aquatic fishes perform respiration with the help of gills. These may be internal gills and that to the exterior as external gill slits. Five pairs of gills are present in shark. The type of respiration performed by the gills is called bronchial respiration. Lungs arise in the embryo as an endodermal diverticulum from the ventral wall of the pharynx. The diverticulum soon divides into two parts, which form right and left kings. A wind pipe or trachea connects the lungs with the pharynx. Anterior part of the trachea is modified into the larynx. The larynx communicates with the pharynx by a slit-like aperture the glottis. The larynx functions as sound producing organ in tetrapods except in birds. So the larynx contains a pair of elastic membranes known as the vocal cords. The respiration performed by lungs is called pulmonary respiration.

Fish and Frog –respiratiory system-similarities and diffierences

Scoliodon (Shark)   Rana ( frog)
1.Scoliodon performs branchial respiration.   1.Rana is adapted for branchial (Tadpole) as well as pulmonary respiration.
2.Respiration is performed by the gills. Skin, and bucco-pharyngeal lining are not respiratory.Lungs are absent.   2.Respiration is performed by the skin,bucco-pharyngeal lining and lungs.There are no gills in the adult.
3.It possess five pairs of internal gills located in the pleural walk of the pharynx. The external gills are absent. But gills are exposedoutside as external gill-slits.   3.Tadpole larva possesses three pairs of external gills in the early stages which are present at the junction of head and trunk. Later these are replaced by three pairs of internal gills.The adult possess the lungs.
4.Respiratory tract consists of mouth, buccal cavity, pharynx, internal branchial apertures, branchial pouches and external branchial apertures.   4.Respiratory tract consists of external nostrils, nasal chambers, internal nostrils, bucco-pharyngeal cavity, glottis and laryngo tracheal chamber. Which open into the lungs.
5.External nostrils, nasal chambers ! are not used in respiration. Inter¬nal nostrils glottis, and laryngo tra¬cheal are absent.   5.Mouth is not used in respiration. Branchial apertures and pouches are absent.
6.The gills have cartilagenous gill I arches to support the gill-axes.   6.Such type of organs are absent.
7.The gills are endodermal in origin and are formed by the outpushing of hypoblast.   7.Lungs are also endodermal in origin.
#####################################   8.Such an arrangement is absent in the adult frog.
9.Generally these are pairs of gill pouches one pair of hemi branch¬es on the posterior side of the hy-oid arch and four pairs of holo branches on the 1st, 2nd, 3rd and 4th branchial arches. The 5th branchial arch is abranch.   9.Absent in the adult frog.
10.The gills-remain in well within the gill pouches.   10.Absent in the adult frog.
11.Absent in shark.   11.The larynx and trachea united and form into laryngo tracheal chamber which is supported by three cartilages a cricoid and a pair of lateralary tenoids.
12.Lungs are absent in shark.   12.Paired lungs are simple, thin-walled elastic sacs having on the inner surface low ridges the septa enclosing the alveoli.
13.No such arrangement is present.   13.The septa and alveoli increase the area coming in contact with air. Exchange of gases takes place all over the inner surface of the lungs.
14.During breathing, the buccal flow is lowered by coracohyal and coracobrachial muscles and raised by constrictor muscles.   14.During breathing, the bucco-pha-ryngeal flow is lowered by the sternohyal muscles and raised by the petrohyal muscles.
15.Water enters the respiratory tract through the mouth and leaves through the external branchial apertures. During this water flow, the dissolved oxygen in the water is absorbed and supplied to the blood in the gills.   15.Air enters the respiratory tract through the external nostrils and leaves also through the external nostrils after providing oxygen to the blood in the buccopharyngeal lining and lungs.
16.The blood is transported to the gills by five pairs of afferent branchi¬al vessels.   16.The blood is transported to the lungs, skin and bucco pharyngeal cavity by the respective arteries.
17.Oxygenated blood from the gills is supplied to the body by nine pairs of efferent branchial vessels.   17.Oxygenated blood from the lungs, skin, and buccopharyngeal cavity is transported to the heart by the respective veins.