Question

Digestive system is complete where
A. All the digestive glands are present
B. Both anus and mouth are present
C. Digestion occurs in the cavity of the digestive tract
D. Digestion occurs in the individual cells lining the digestive tract

Answer 1

Hint: The digestive system in an organism is responsible for breaking food into simpler forms and absorbing energy from it. The digestive system is considered complete in those organisms which have two kinds of opening for the digestive tract.

Complete answer:
In the animal kingdom, two kinds of digestive tracts are present-
a. Incomplete digestive tract
b. Complete digestive tract
The incomplete digestive tract is the tract which has only one kind of opening. It is found in the organisms with cellular aggregate or blind sac level of the organization.
The complete digestive tract is the tract which has two kinds of openings. One is the mouth from which the food enters the alimentary canal or the body and another one is the anus from which the fecal matter takes exit from the alimentary canal or the body. It is found in organisms with a tube within the tube level of body organization.

Organisms with the tube within tube level organization are further classified into two categories, Protostome and Deuterostome. In protostomic organisms, the mouth forms first and then the anus. In deuterostomic organisms first anus forms and then mouth.
Thus, based on the above information we can conclude that the digestive system is complete where both anus and mouth are present.
Hence, the correct answer is option (B).

Note: At the cellular level of organization organisms have an opening that acts as a mouth and the pores act as the anus. There is no alimentary canal. Digestion occurs in the body cavity. In the organisms with a Blind sac level of organization, a single opening acts as both mouth and anus. Both of these body organizations do not show a complete digestive tract.