Answer briefly:  (a) Why are villi present in the intestine and not in the stomach?(b) How does pepsinogen change into its active form?(c) What are the basic layers of the wall of alimentary canal?(d) How does bile help in the digestion of fats? 

(a) The digested food is passed into the intestine for absorption. The villi are present here to increase the surface area for the absorption of the digested food. (b) The inactive pepsinogen is converted into its active form called pepsin by the action of the hydrochloric acid which is present in the gastric juice. (c) The walls of the alimentary canal possess four layers. They are:  1.serosa  2.muscularis  3. sub-mucosa and  4. mucosa. (d) Bile contains bile pigments, bile salts, cholesterol, and phospholipids. These help in the emulsification of fats where the fat molecules are broken down into small micelles.