The correct answer is option 'C' - Balanoglossus.

Balanoglossus is a marine animal that belongs to the phylum Hemichordata. It is commonly known as acorn worm. One of the unique features of Balanoglossus is the presence of a structure called the stomochord or buccal diverticulum.

The stomochord is a tubular structure that extends from the pharynx to the digestive tract in Balanoglossus. It acts as a supportive structure and is considered a primitive precursor to the vertebrate chorda dorsalis or notochord. The stomochord is lined with endodermal cells and surrounded by connective tissue.

The stomochord plays a crucial role in the feeding and digestion process of Balanoglossus. It helps in the transport of food from the mouth to the digestive tract. The walls of the stomochord are muscular and can contract to facilitate the movement of food. It also helps in the secretion of digestive enzymes.

Balanoglossus has a unique feeding mechanism known as enteropneust feeding. It burrows into the sand or mud, leaving only its proboscis exposed. The proboscis is a muscular structure that is used to capture and transport food particles into the mouth. The stomochord assists in the movement of food from the proboscis to the digestive tract.

The stomochord is not present in Amphioxus (option 'A') and Herdmania (option 'B'). Amphioxus, also known as lancelet, is a marine chordate belonging to the subphylum Cephalochordata. It lacks a stomochord and notochord. Herdmania, also known as sea squirt, is a marine invertebrate belonging to the phylum Chordata. It possesses a notochord but does not have a stomochord.

In conclusion, the stomochord or buccal diverticulum is present in Balanoglossus (option 'C'). It is a unique structure that aids in the feeding and digestion process of this marine animal.

Characteristic feature of hemichordates....