It consists of the following parts:
At the base of proboscis stalk a wide circular opening, mouth is situated ventrally and covered by collarette.
It has two sets of muscle fibers , the radial fibers to open it and the concentric fibers to close it. Mouth leads into buccal cavity.
(2) Buccal cavity:
It lies in collar region and its epithelial wall contains glandular goblat cells. Anteriorly it forms hollow buccal diverticulum and posteriorly it extends up to the collar trunk septum and leads into pharynx.
It lies in the branchial region of the trunk. Its wall has lateral constrictions projecting into its lumen as ridges called parabranchial ridges.
These ridges incompletely divide the pharynx into a dorsal respiratory or branchial portion and a ventral digestive portion. Ventral digestive portion lined with ciliated epithelium with gland cell, helps in food concentration.
Behind the last pair of gill slits the pharynx continues into the short oesophagus. The dorsal part of the oesophagus is called post branchial canal which possesses thick folded and glandular epithelium. Posterior part of oesophagus reduces in diameter and has deeply furrowed epithelium.
It occupies hepatic and post hepatic region of trunk. Intestine also extends intestinal caecum corresponding to the hepatic caeca of this region. Post hepatic region of intestine is connected with the ventral body wall by the pygochord.
Intestine opens to the exterior by a terminal circular aperature, the anus at the tip of the trunk. It is often surrounded by a sphincter muscle.
Food, feeding and digestion:
Balanoglossus is a cilliary feeder. Its food comprises of microscopic organisms and organic particles present in water and the bottom sand in which it makes its burrows.
A respiratory cum food water current is set up by the lateral cilia of gill slits. It enters the mouth, passes through the buccal cavity, pharynx, gill slits and branchial sacs and leaves through the gill pores.
The food, present in the water current is collected by the mucus, secreted by the proboscis gland cells in the form of mucus string.
This mucus string is directed towards the pre-oral ciliary organ at the base of the proboscis by the cilia covering the proboscis.
Then it goes back to the mouth. U-shaped pre oral ciliary organ tests the quality of food and water entering the mouth.
Backward movement of food through the alimentary canal is maintained by the cilia lining of its walls.
Digestion takes place by enzymes secreted by gland cells of the pharynx, oesophagus and hepatic region of the intestine.
Exact process of digestion in Balanoglossus is not known. Undigested substances, along with sand and silt pass out through the anus as “Castings”.