Marsh frog internal structure and amphibian reproduction

The frog’s digestive tract resembles a fish’s: mouth cavity (tongue, small teeth), pharynx, gullet, stomach, intestine; the hind gut widens to form the cloaca. It breathes with lungs (simple sacs) and skin: when the throat is lowered air enters the lungs, when raised it leaves. The heart is three-chambered: two atria and one ventricle. In the systemic circuit blood goes from the ventricle to the organs and back to the right atrium; in the pulmonary circuit it goes from the ventricle to the lungs, takes up oxygen and returns to the left atrium. Some blood also goes to the skin, picks up oxygen there and comes back through veins to the right atrium. Both atria contract together, so blood mixes partly in the ventricle. Excretory organs are two kidneys and ureters, and urine leaves via the cloaca. The frog is cold-blooded: temperature follows the surroundings and in winter it is torpid. The brain is better developed than in fishes, with cerebral hemispheres, while the cerebellum is weak (slow movement). The sense organs are adapted to land: eyelids, a middle and inner ear (the eardrum and an auditory bone carry sound).