The starfish consumes its prey outside its body, by using tiny suction-cupped tube feet and sack-like cardiac stomach THAT emerges from the mouth to envelop the prey and digest it. 'THAT' in above sentence stands for:

In the sentence structure, the relative clause "that emerges from the mouth" follows immediately after "sack-like cardiac stomach," making it the antecedent. The stomach is what emerges to envelop and digest prey. "Starfish" is the subject but not what "that" refers to, and "feet" and "prey" are other nouns in the sentence but not the correct referent.