Digestive system of Balanoglossus

Balanoglossus is an exclusively marine animal, ocean-dwelling worm. This cylindrical entity inhabits sand burrows and serves as an evolutionary link between vertebrates and invertebrates, hence is a Hemichordate. It is a soft-bodied, worm-like entity whose size varies between 2 and 3 mm. The surface of their body is usually ciliated and the body is divisible into three regions – Proboscis, Collar and trunk.

Balanoglossus – Digestive system

The digestive system of Balanoglossus exhibits a long alimentary canal. This straight tube is provided along with associated glands where in the alimentary canal begins anteriorly with the mouth – circular and wide located at the ventral side in a groove between the proboscis stalk and collarette and ends posteriorly with the anus – circular aperture towards the extreme end of the trunk.

The alimentary canal can be segregated into four sections – Oesophagus, Buccal tube, Pharynx and the intestine. The walls consist of ciliated epithelium externally lined by basement membrane and lack muscle fibers.

Parts of Digestive system of Balanoglossus

Following table shows different parts that make up the digestive system of the Balanoglossus:

Mouth

This permanent pore is present ventrally between the proboscis and collar. Found at the bottom of the proboscis, ventrally located between collarette and proboscis stalk, to be precise. It can be open or closed as per recent studies. The mouth is enabled with 2 sets of muscle fibers wherein radial fibers cause opening and concentric fibers cause its closing. It leads to the buccal cavity.

Buccal cavity

Mouth ends in the short buccal cavity. The lining of this cavity is with glandular and ciliated cells. A stiff, hollow buccal diverticulum projects from the ceiling of this cavity which extends as a stomochord into the proboscis coelom. It extends till the collar-trunk septum posteriorly leading to the trunk as the pharynx.

Pharynx

Found in the branchio-genital section of the trunk, the walls comprise a longitudinal constriction through each of the lateral sides which extend into the lumen as parabranchial ridges. It consists of tall columnar cells.

The parabranchial ridges partially split the pharynx into the brachial portion (dorsal pore pharynx) and digestive portion (the ventral digestive pharynx).

The dorsal respiratory pharynx is dorsolaterally perforated by two rows of U-shaped gill-slits used for respiration and passing out water through gills on the wall of the body. The ventral food collecting pharynx is covered by ciliated glandular epithelium which concentrates the food entities found in water entering into the chamber, digestion and food absorption.

Oesophagus

The terminal pair of gill-slits of the pharynx leads to a tiny part of the alimentary canal, the oesophagus. The ventral and dorsal division of the pharynx extends some more into the oesophagus. Here, the posterior section of oesophagus diminishes in diameter and possesses deep furrowed epithelium whereas the dorsal section is the post-branchial canal having folded, thick glandular epithelium.

Intestine

Intestine is found behind the oesophagus as a long straight tube extending along the trunk. It can be distinguished into the hepatic and the post-hepatic caecae of the trunk. The hepatic area is vascular wherein the epithelial cells are dark green/brown and the dorsal wall forms several eminent sacculations known as the hepatic caecae. It protrudes the wall of the body causing them to be visible. On the other side, the post-hepatic area is associated with the ventral body wall of the pygochord. It is in the form of a simple tube comprising a pair of dorso-lateral grooves run along by ciliated epithelial cells.

Anus

Intestine leads posteriorly into the anus. The alimentary canal ends with the anus at the end of the trunk which typically comprises the sphincter muscles.

Digestive Mechanism of Balanoglossus

The food of this ciliary feeder constitutes microscopic entities and particles found in the bottom of the sand wherein it burrows and in water. A current of water is established by the lateral cilia which lines the gill-slits entering the mouth, travels through the buccal cavity, then the pharynx, and gill-slits all the way to the branchial sacs and finally exit via the gill pores. This is referred to as the respiratory-cum food current. Courtesy this pathway, some particles enter the mouth, some other food particles are entangled in the mucus of the proboscis which covers it.

The epithelial gland cells of the proboscis epithelium secrete mucus. This cilia which covers the proboscis leads the mucous string having food particles to the pre-oral ciliary structure at the bottom of the proboscis. The mucous string is put back into the mouth by the action of the proboscis cilia from this string along with the aid of the main water current which enters the mouth.

The U-shaped depression at the base of proboscis provided ventrally is lined by tall epidermal cells with long cilia, it is the pre-oral ciliary organ which assesses the water and food quality which enters the mouth. The rejected particles are passed back over the collar through the ventral section of the collarette covering the mouth.

Food in pharynx passes through the ventral digestive portion. Hepatic region (intestine), gland cells (pharynx) and Proboscis produce enzymes causing food digestion.

You just learnt about the Digestive system of Balanoglossus through these various organs. Explore more related topics for NEET, at BYJU’S.

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