Important parts of digestive system in herdmania are mentioned below:
In Herdmania it is coiled and complete and contains the following parts.
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(1) Mouth:
It forms the opening of branchial siphon marking the anterior end of the body. It is guarded by four lobes derived from the test.
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(2) Buccal Cavity:
Mouth leads into a laterally compressed cavity of branchial siphon, called buccal cavity or stomodaeum.
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A strong branchial sphincter present at the base of siphon which regulates the opening of mouth. A circlet of highly branched delicate branchial tentacles, richly supplied by nerves is also found at the base of siphon.
These are 64 in number and of four different sizes like 8 large (5 mm) 8 medium (2.5 mm) 16 small (1.5 mm) and 32 very small (0.5 mm).
The free ends of tentacles meet at the centre forming a sort of strainer which prevents entry of larger food particles into pharynx.
(3) Pharynx:
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Buccal cavity leads into the pharynx, 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 separated from the branchial sac by two circular thin parallel ciliated ridges called anterior and posterior peripharyngeal bands, enclosing a narrow ciliated peripharyngeal groove.
The anterior peripharyngeal band is a complete ring and mid dorsally in front of it lies a smaller 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 and is also known as branchial basket. Because its wall is perforated by numerous gill slits or stigmata, each side of branchial sac bears about 200,000 stigmata arranged in several transverse rows.
The epithelium lining of the stigmata bears long cilia called lateral cilia. Pharyngeal wall is divided into stigmatic rectangular areas by longitudinal and transverse bars, each area having 5 or 6 stigmata.
All the bars of the wall are highly vascular arid contain their corresponding blood vessels. Some other structures also are associated to branchial sac that has their role in digestion.
(i) Trabeculae:
Trabeculae are hollow strands, containing a blood vessel, connecting outer wall of branchial sac to the mantle.
The inner wall of branchial sac bears 9 to 10 longitudinal branchial folds to increase its surface area.
(ii) Dorsal lamina:
It is a thin flap or fold extended mid-dorsally from the posterior pharyngeal band. It is 1 to 1.5 cm long, bears 20 to 30 conical tapering tongue like processes called languets in a row.
These languets hang down from dorsal lamina into the cavity of the branchial sac. The languets are covered by ciliated epithelium. They form a sort of groove for conducting food.
(iii) Endostyle:
This is a shallow mid-ventral groove lying on the floor of the banchial sac. Anteriorly it joins to the peripharyngeal groove and the marginal folds of endostyle merge with the posterior peripharyngeal band.
While posteriorly these 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. Endostyle is homologous to the hypopharyngeal groove of cephalochordates and thyroid glands of vertebrates.
(iv) Oesophageal area:
The posterior most region of branchial sac has a small circular oesophageal area. Its opening is guarded by two semi-circular lips. This area is devoid of blood vessels, folds and stigmata.
4. Oesophagus:
It is very short, curved and thick walled tube, contains four longitudinal ciliated grooves, connects branchial sac to the stomach.
5. Stomach:
It is wider than the oesophagus, thin walled, sphinctered at both ends and has a smooth inner lining.
6. Intestine:
It 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.
7. Rectum:
Intestine leads into the rectum, internally lined by cilia. It curves dorsally to open into the atrium or cloaca through the anus. Anus is guarded by four lips.
8. Cloaca:
The atrium or cloaca leads into the atrial siphon and opens outside through the atrial aperture.