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The cardiac events that occur from the beginning of one heart beat to the beginning of the next are called cardiac cycle. 1.To begin with, all the four chambers of heart are in a relaxed state, i.e., they are in joint diastole. As the tricuspid and bicuspid valves are open, blood from the pulmonary veins and vena cava flows into the left and the right ventricle respectively through the left and the right atria. The semilunar valves are close at this stage. 2.The SAN now generates an action potential which stimulates both the atria to undergo a simultaneous contraction – the atrial systole. This increases the flow of the blood into the ventricles by about 30 percent. 3.The action potential is conducted to the ventricular side by the AVN and AV bundle from where the bundle of HIS transmits it through the entire ventricular musculature. This causes the ventricular muscles to contract. (ventricular systole), the atria undergo relaxation (diastole), coinciding with the ventricula

Fishes are mainly aquatic, jaw bearing vertebrates. They have gills for respiration and paired fins for swimming. Fishes are divided in to evolutionary lines – 1.Cartilaginous fishes or Chondrichthyes – it is also called Elasmobranchs. It includes – sharks, rays, skates, Chimaeras. 2.Bony fishes or Osteichthyes -   it is true bony fishes. It includes – Carp, Perch, Bass, Trout, Catfish and Sucker fish. There are some differences between these – Characters Cartilaginous fishes Bony fishes 1.Habitat Mostly marine Marine & pure water both 2.Size Generally dorsoventrally flat Generally bilaterally flat 3.Caudal fin Hetero-cercal Homocercal or Diphycercal 4.Pelvic fin Generally posterior, and forms claspers in male. Generally anterior, claspers absent. 5.Mouth Present on the ventral side of head Present on the anterior part of head 6.Gill pores