Digestive system of Balanoglossus (Phylum- Hemichordates)

(i) Alimentary canal:

In alimentary canal of Balanoglossus is a straight pipe-like structure situated antero-posteriorly in the body. It is divided into four parts--

(1) Mouth:

It is a broad circular pore situated in between proboscis stalk and collarette on the ventral surface. It is permanently opened.

(2) Buccal tube and Buccal cavity:

The short buccal cavity lies in the collar region. It is lined with ciliated and glandular cells. From its roof arises a stiff, hollow buccal diverticulum extending into the proboscis coelom. Posteriorly it extends up to the collar-trunk septum and then extends into the trunk as pharynx.

(3) Pharynx:

It is situated in the bronchial region of the trunk. One both of the lateral sides of its wall lateral longitudinal constrictions are found. Each of these lateral longitudinal constriction protrudes one in the pharynx and are known as parabranchial ridges. These are lined by long columnar cells. These lateral longitudinal constrictions and parabranchial ridge divide the pharynx in an upper clome or branchial part and the second lower digestive part.

(i) Branchial part:

It is an upper part of pharynx related with respiration. A row of gill pores is situated on each of its dorsolateral surfaces. Each gill pore is U shaped and opens in a branchial sac. This has been well discribed in respiration process.

(ii) Digestive part:

It is lower part of pharynx which is concerned with digestion of food and its absorption. Its inner wall is lined by ciliated epithelial cells which contain glandular cells also.

(iii) Oesophagus:

Behind the last pair of gill-slits the pharynx continues into the oesophagus. The dorsal and ventral division of the pharynx continues for some distance into the oesophagus; in this region, the dorsal part of the oesophagus is called post-branchial canal which possesses thick, folded and glandular epithelium. The posterior part of the oesophagus reduces in diameter and has deeply furrowed epithelium.

(4) Intestine:

Behind the oesophagus is an intestine. It occupies the hepatic and post-hepatic regions of the trunk. The hepatic region of the intestine is highly vascular. Its epithelial cells are dark green or dark brown and its dorsal wall forms numerous prominent sacculations called hepatic caeca which push the body wall outwards and are, thus, visible externally.

The post- hepatic region of the intestine is connected with the ventral body wall by the pygochord. The intestine has the form of a simple tube and bears a pair of dorso-lateral grooves lined by tall epithelial cells bearing long cilia.

(5) Anus:

Posteriorly the intestine opens out through the anus situated at the extreme hind end of the body. The anus often possesses sphincter muscles.

(ii) Food and feeding mechanisms:

Balanoglossus is a ciliary feeder animal. Its main food are microscopic organisms and inorganic particles which are found on bed sand and in water around its burrow. Lateral cilia lining the gill pores produce water current which enters into mouth cavity through mouth opening and moves through pharynx, gill slits and gill sacs to come outward through gill pores. This water current is known as respiratory cum food water current. It helps both in food absorption and respiration.

Some food particles coming through the water current directly enter into mouth whereas some get entangled into mucous membrane after coming in contact with proboscis. This mucous is secreted by glandular cells present in proboscis epithelium. These cilia lining the proboscis, pass the food particles containing mucous membrane towards the pre-oral ciliary organ at the base of proboscis in the form a mucous chord. The pre-oral ciliary organ now pushes particles on mucous choro towards posterior part of mouth.

There are U shaped epidermal depressions at the base of the ventral layer of proboscis. These are surrounded by long epidermal cells containing long cilia. This Structure is known as pre-oral ciliary organ. These organ examine the food particles and water coming in the mouth. After testing the food particles not useful for entering into mouth are stopped to enter into mouth by lower part of collarette. This action is accomplished by shutting the mouth and the unwanted food particles do not enter and pass posteriorly through the upper collar region

(iii) Digestion:

Cilia present in the alimentary canal continously push the food backward inside this canal. Food moves through lower enzymatic region of pharynx where some enzymatic glandular cells present there secrete and mix the enzymes in the food. Such enzymes help in the digestion of the food. There is absorption of digested food in the intestine. Rest of the sand and silt particle coming through the water current are expelled outside with undigested food through the anus.