Digestive System of Balanoglossus (Hemichordata)
The alimentary canal in Balanoglossus is a complete and straight tube running between the mouth and anus. It is supported throughout its length by the dorsal and ventral mesentries. Its wall is made up of ciliated epithelium covered externally by a basement membrane, but peculiarly, muscle layers are absent. Alimentary canal comprises of (i) mouth, (ii) buccal cavity, (iii) pharynx, (iv) oesophagus, (v) intestine and (vi) anus.
It is a wide and circular opening is situated ventrally in a groove between the proboscis stalk and collarette. It can be closed or opened and does not remain permanently open. It has two sets of muscle fibres, the radial fibres to open it and the concentric fibres for closing it. The mouth leads into buccal cavity.
The buccal cavity is short and occupies the collar region. Its epithelial wall contains glandular goblet cells. Anteriorly its dorsal walls form a short, stiff and hollow buccal diverticulum that projects into the proboscis coelom. Posteriorly it extends up to the collar-trunk septum behinds which it continues into the pharynx.
It lies in the branchial region of the trunk. Externally its wall bears a longitudinal constriction along each lateral side. These lateral constrictions project into its lumen as ridges, called parabranchial ridges, consisting of tall columnar cells. These ridges incompletely divide the pharynx into a dorsal respiratory or branchial portion and a ventral digestive portion. The dorsal branchial portion is perforated dorso-laterally by two rows of U-shaped gill-slits, and is concerned with respiration. The ventral digestive portion, lined with ciliated epithelium with gland cells, helps in food concentration.
Pharynx continues into the short oesophagus behind the last pair of gill-slits. The dorsal and ventral divisions of pharynx continue for some distance into oesophagus. In this region, the dorsal part is called post-branchial canal which possesses thick, folded and glandular epithelium. The posterior part of oesophagus reduces in diameter and has deeply furrowed epithelium.
It occupies the hepatic and post-hepatic regions of 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 sacculations called hepatic caeca. The intestinal sacculations correspond with those of the body wall. The post-hepatic region of intestine is connects with the ventral body wall by the pygochord. It is a simple and straight tube bearing a pair of dorso-lateral grooves lined by tall epithelial cells with long cilia.
Posteriorly, the intestine opens to the exterior by a terminal circular aperture, the anus, at the tip of the trunk. It is often surrounded by a sphincter muscle.
Food, Feeding and Digestion
Balanglossus is a ‘ciliary feeder’. Food generally consists of microscopic organisms and organic particles present in water and the bottom sand in which it makes its burrows. The lateral cilia lining the gill-slits set up a current of water which enters through the mouth, takes its course through the buccal cavity, pharynx, gill-slits and branchial sacs, and leaves through the gill pores. This is the respiratory-cum-food current. Some food particles directly enter the mouth with this current while some come in contact with the proboscis and get entangled in the mucus that covers it. The mucus is secreted by the gland cells of the proboscis epithelium. Cilia covering the proboscis direct the mucous string, containing food particles, towards the pre-oral ciliary organ at the base of the proboscis. From here the mucous string is passed back into the mouth by the action of the proboscis cilia, aided by the main water current entering the mouth. Organic particles present in the sand are ingested directly along with the latter at the time of burrowing.
The U-shaped pre-oral ciliary organ, at the base of proboscis stalk, tests the quality of food and water entering the mouth. Undesirable substances are prevented from entering the mouth by the ventral part of the collarette, which does so by covering the mouth. Thus, the rejected particles, instead of entering the mouth, pass back over the collar.
Backward movement of food through the alimentary canal is maintained by the cilia lining present in its walls. In the pharynx, the food moves through the ventral digestive portion. Digestion is brought about by enzymes secreted by gland cells of the pharynx, oesophagus and hepatic region of the intestine. The exact process of digestion in Balanoglossus is not known yet. Undigested substances, along with sand and silt, pass out through the anus as ‘castings’.
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