Digestive system of Herdmania

Digestive tract of Herdmania is coiled and complete, beginning at the mouth and terminating at the anus. The following parts form the alimentary canal.

Mouth

The mouth or branchial aperture opens on the top of the branchial siphon masking the anterior end of the body. It is bordered by four lips or lobes formed by the test and leads into the branchial siphon.

Buccal cavity

The short narrow and laterally compressed cavity of branchial siphon, lined by epidermis, is called the stomodaeum or buccal cavity. A strong branchial sphincter present near the base of the siphon regulates its opening. The base of the siphon is provided with a circlet of highly branched, delicate branchial tentacles richly supplied with nerves. Their number is about 64 which are broadly in four sizes, such as 8 large (5 mm), 8 medium (2.5 mm), 16 small (1.5 mm) and 32 very small (0.5 mm).

A branchial tentacle is sickle-shaped, attached by its broad base. Its upper and thicker border bears numerous paired lateral branches, called tentaculets, each in turn bearing smaller secondary and tertiary branches. The lower border of tentacle is thin, lobed, notched and covered by glandular epithelium. The free ends of tentacles meet at the centre forming a sort of strainer which prevents entry of larger food particles into pharynx.

Pharynx

Buccal cavity or branchial siphon is followed by the pharynx. It occupies the major part of the body or atrial cavity. It is differentiated into a prebranchial zone and a branchial sac.

(a) Prebranchial zone. It is the smaller anterior region having smooth walls without folds, cilia and stigmata or gill-slits. It is demarcated from the branchial sac by two circular, thin, parallel and ciliated ridges, called anterior and posterior peripharyngeal bands, enclosing a narrow ciliated peripharyngeal groove. The two bands bear larger cilia while the groove has smaller cilia.

The anterior peripharyngeal band is a complete ring and mid dorsally in front of it lies a swollen dorsal tubercle made of two spiral coils. The posterior peripharyngeal band is interrupted mid-dorsally by the dorsal lamina and mid-ventrally by the endostyle.

(b) Branchial sac. It is the larger posterior region of pharynx. It is also known as the branchial basket because of its looks, as its lateral walls are perforated by numerous elongated gill-slits of stigmata through which the cavity of pharynx communicates with the atrial cavity. Each side of branchial sac bears about 200,000 stigmata arranged in several transverse rows.

The epithelium lining the stigmata bears long cilia, called lateral cilia. The pharyngeal wall consists of a network of broad longitudinal and transverse bars at regular intervals, enclosing many square or rectangular areas, called stigmatic areas, each having 5 or 6 stigmata. The pharyngeal wall is richly vascular and the bars contain their corresponding blood vessels, i.e., internal longitudinal and external transverse vessels. The adjacent stigmata are also separated from each other by thin longitudinal intersegmental bars and vessels. Besides, a thin transverse intrastigmatic bar and vessel transversely cross through each row of stigmata.

Trabeculae :- The inner wall of branchial sac becomes folded longitudinally to increase its surface area. The number of branchial folds is 9 or 10 on either side. The inner surface is nonciliated. The outer wall of branchial sac is connected to mantle by several hollow strands, called trabeculae, each containing a blood vessel.
Dorsal lamina :- A thin flap or fold, I to 1.5 cm long, is suspended from the short mid-dorsal border of the roof of branchial sac. It is called the hyperpharyngeal hand or dorsal lamina. It extends from the posterior peripharyngeal band to the right lip of oesophageal opening. A row of 20 to 30 conical, tapering tongue-like processes, the languets, hang down from dorsal lamina into the cavity of the branchial sac. The languets covered by ciliated epithelium. In the living animal, the dorsal lamina and its languets are bent to the right side forming a sort of gutter for conducting food.
Endostyle :- Similarly, a shallow longitudinal mid-ventral groove, called endostyle, is present on the floor of the branchial sac. Anteriorly, the endostylar groove is continuous with the peripharyngeal groove, while the marginal folds of endostyle merge with the posterior peripharyngeal band. Posteriorly, the groove terminates before reaching oesophageal opening, while its folds reach up to oesophageal opening as thin retropharyngeal folds. Structurally, endostyle consists of five longitudinal ciliary tracts (1 median, 2 lateral pairs) alternating with four longitudinal tracts of mucus-secreting glandular cells. The cilia of the median tract are the longest of all. The endostyle of urochordates is homologous with the hypopharyngeal groove of cephalochordates and thyroid glands of vertebrates.
Oesophageal area :- The posteriormost region of branchial sac has a small circular oesophageal area made of two semicircular lips guarding the oesophageal opening. This area is devoid of blood vessels, folds are stigmata.

Oesophagus

The branchial Sac postero-dorsally leads into the oesophagus which is a very short, curved and thick-walled tube opening into stomach. Its narrow lumen is produced into four longitudinal ciliated grooves for the passage of food.

Stomach

It is wider than oesophagus, Thin walled, sphinctered at both ends, has a smooth inner surface, and is surrounded on either side by the right and left lobes of liver.

Intestine

The stomach leads into the intestine which is a thin-walled, U-shaped tube formed by a proximal, ventral or descending limb and a distal, dorsal or ascending limb, both united anteriorly. The intestinal loop, thus formed, encloses the left gonad.

Rectum

Posteriorly the terminal part of intestine leads into a short, narrow tube, the rectum, internally lined by cilia. It curves dorsally to open into the atrium or cloaca through the anus which is bounded by four lips.

Cloaca

The atrial cavity or cloaca leads dorsally into the atrial siphon which opens to the outside through the atrial aperture. The atrial siphon is lined internally by ectoderm and thus represents the proctodaeum.

Digestive glands

Liver

Liver of Herdmania is a large dark brown bilobed digestive gland made of larger left lobe and a smaller right lobe attached on either side of the stomach. Liver is composed of a large number of tubules or caeca embedded in a connective tissue matrix containing blood sinuses. Tubules unite to form 11 or 12 hepatic ducts which open independently into the stomach. Liver secretion contains a strong amylase, a protease and a mild lipase.

Pyloric gland

It consists of a large number of brandling tubules in the walls of stomach and intestine. They open by a single duct into the middle of the proximal limb of intestine. The pyloric gland of Herdmania probably performs a dual function, that of a vertebrate pancreas and of an excretory organ.

Food, feeding and digestion

Food

Due to sedentary habit, Herdmania is a ciliary or filter feeder. The food consists of microscopic planktonic organisms such as algae, diatoms, infusorians, etc.

Feeding or food collection

Outward beatings of lateral cilia lining, the stigmata set up a . constant water current which enters through the mouth and buccal cavity into the pharynx, passes through stigmata into the atrial cavity and leaves the body through atrial siphon (cloaca) and atrial aperture. The branchial tentacles, forming a sort of sieve or strainer, prevent larger particles but allow minute food particles to enter the pharynx along with the water current. The tentacles probably also act as chemoreceptors and keep out the impurities present in sea water. The larger particles and impurities are thrown out of the mouth by producing a strong reverse current.

The lateral cilia of stigmata retain the food particles inside the branchial sac, which settle on its wall. Enormous quantity of viscid mucus secreted by the gland cells of endostyle is whipped up along the inner lateral walls of branchial sac by the lashing movements of the endostylar cilia. The food particles entangled with this mucus are also shifted dorsally by the frontal cilia that beat upwards. The food-laden mucus finally reaches the dorsal lamina, rolled up into a cylindrical mass.

Here, it is driven backwards by ciliary action in the form of a string or coid, in the groove formed by the curved languets of dorsal lamina and passes into the oesophagus aided by the oesophageal lips. From oesophagus the food passes into the stomach.

Digestion

In stomach, food is digested by the enzvmes present in the liver secretion. Amylase hydrolases starch into maltose, protease breaks down proteins. and lipase acts on fats. Secretion of pyloric gland has an accessory digestive function similar to that of pancreas. Digestion is completed and the digested food absorbed in the intestine.

The undigested food material passes into rectum and driven out by rectal cilia into cloaca through anus as a much coiled faecal cord with mucus.

It is expelled out forcefully up to a distance of 10-12 cm through atrial aperture by the sudden contractions of the body. Reserve food in the form of concentrically striated starch-like granules is present in the liver and the walls of oesophagus, stomach and intestine.