(a) Why is the small intestine in herbivores longer than in carnivores?
(b) What will happen if the mucus is not secreted by the gastric glands?
(c) What causes movement of food inside the alimentary canal?

(a)  Herbivores depend on plants for their nutrition. Example: Deer, cow, goat.  These plants contain complex carbohydrates like cellulose and hemicellulose that need to be broken down, hence they need a longer intestine. Carnivores do not eat plants, they depend upon other animals for their nutrition. Example: Lion, Tiger, Cat. So they can digest their food easily as compared to herbivores. Hence, the small intestine in herbivores is longer than in carnivores.

(b) Mucus protects the stomach wall from hydrochloric acid. If the mucus is not secreted by the gastric glands then the gas will damage the inner lining of the stomach and lead to ulcers in the stomach.

(c) Peristalsis causes the movement of food inside the alimentary canal. 

The contraction and relaxation of muscles of the esophagus, intestine, and stomach are called peristaltic movement. The movement is involuntary and is mandatory for the movement of food from the mouth and bowels through the anus.