What causes the movement of food inside the alimentary canal?

Why is blood circulation in the human heart called double circulation?

Why is the rate of breathing in aquatic organisms much faster than in terrestrial organisms?

How do desert plants perform the photosynthesis process even though their stomata remain closed during the daytime?

Why is it advised to use iodised salt in our diet?

$(a)$ Name the extensively coiled structure of the alimentary canal.
$(b)$ Compare the length of the small intestine in herbivores and carnivores.