Digestive system of Pila: An Apple Snail.

Digestive system of the Pila: an Apple Snail.

The digestive system of An apple Snail comprises following three parts.

1. A tubular alimentary canal:
2. A pair of the salivary gland.
3. A large digestive gland.
4. Alimentary canal.

1. Alimentary canal:

The alimentary canal of pila is coiled tube extending from a mouth and terminating at the anus. Its anterior parts are specially modified. The entire canal may be divided into three regions.

1. The foregut or stomodaeum including the buccal mass and oesophagus.
2. The midgut or mesenteron consisting of stomach intestine, and
3. The hindgut or proctodaeum comprising the rectum.

[I] Foregut:

It includes the mouth, buccal mass and oesophagus.

1. Mouth:

In the mouth of Pila, there is the absence of true lips but the plicate edges alone serve as secondary lips. It is narrow, vertical slit situated at the end of snout

2. Buccal cavity:

The mouth leads into the chamber known as a buccal cavity. It is lined by cuticle and surrounded by a large, thick-walled, highly muscular and pear-shaped structure, the buccal mass. The wall provided with several sets of muscles for its movement and the movement of radula.it is divided into following four parts:

(a) Buccal musculature.

(b) Vestibule and jaws.

(c) Odontophore.

(d) Radula.

1.Buccal musculature:

These muscles are mainly concerned with the protrusion of lateral cartilage and the protractors are well developed. They included(i)- a medium dorsal, three pairs of anterior dorsolaterally and two pairs of posterior dorsolateral on te dorsal side, and (ii)- three anterior muscles and a pair of long and strong lateral-ventral forward muscles on the ventral surface. It also assists in the protrusion and the depression of buccal mass

2. Vestibule and jaws:

The anterior part of the cavity of buccal mass is known as a vestibule.On the posterior part, the vestibule is a pair of thickening twp jaws handling from the roof of buccal mass. The jaw bear muscles and the anterior edges have teeth-like projections for cutting up vegetable food.

3. Odontophore:

On the posterior part of the buccal cavity, the floor is a large elevated and the thick muscular structure called tongue mass or odontophore. The front part of odontophore has a furrowed subgranular organ which helps in cutting food. It has protractor and retractor muscles and is supported by two pairs of cartilages,

1.a pair of triangular superior cartilages which project into the buccal cavity,

2.And a pair of large S-shaped lateral cartilages.

4. Radula:

The buccal cavity contains a brownish, chitinous, curved, ribbon-like structure, called the radial or lingual ribbon.It is anterior end-bearing a pair of wing-like flaps runs longitudinally over the odontophore and the posterior end is lodged in a band-like 2mm wide glandular sac flexed behind and below the buccal mass. The radula is made of many transverse rows of horny teeth which are characteristics of Mollusca.Each row has seven teeth, two marginal and one lateral tooth on each side and a central or rachidian tooth in the middle, thus, giving a formula 2,1, 1, 1, 2, The radula moves forward and backwards on the odontophore for rising food particles; this movement of radula are called chain saw movement.The teeth are made of chitin which is reinforced by hardened protein thus have sharp cutting projections which act like a file and rasp vegetable food. The teeth of the radula are worn off in front and new teeth are formed all the time of odontoblasts. On the roof of the buccal cavity, above the radula, is a pair of grooved buccal glands which are digested. Radula can cause the radula to wear off at the anterior end, but the loss is made good by regular addition of granular material at the posterior end.

2 Oesophagus:

The buccal mass leads into a narrow and long tube emerging dorsally called oesophagus. From near the origin of the oesophagus arise a pair of round, whitish oesophagal pouches. Running posteriorly, it turns to the left and enters the visceral mass to open into the stomach.

[II] Midgut:

It includes stomach and intestine:

1. Stomach:

The stomach is a rectangular sack of dark red colour heaving a broad U-shaped internal cavity of rose-red colour. It lies on the left side just below the pericardium and runs backwards as a blind pouch on the postero-lateral side of the main whole of the visceral mass.It is differentiated into two chambers.

Cardiac and the pyloric chamber.The cardiac chamber is rounded in appearance and possesses longitudinal folds on its inner surface. The oesophagus opens into it. The pyloric chamber is tubular and has transverse folds on the inner surface. From the pyloric chamber arises a short bag-like caecum but has no crystalline style as found in many gastropods.At the junction of two chambers of the stomach open a duct from the digestive gland.

2. Intestine:

Pyloric muscle is followed by a long and coiled intestine which runs along its anterior edge and further along the digestive gland beneath the posterior renal chamber. It runs backwards into the visceral mass where it makes 2.5 or 3 coils, between the gonad in front and the digestive gland behind, before joining the rectum.

[III] Hindgut:

1. Rectum:

It comprises of a thick-walled tube of alimentary canal which extends into the branchial chamber of the mantle cavity between the ctenidium and genital duct. It enters the mantle cavity and passes downwards to open by an anus on the right of the head. It is external opening, the anus, lies about 6 mm away from the edge of the right nuchal lobe.

2. Salivary gland:

These are two in number and lie on either on the posterior part of the buccal mass and partially cover the oesophagus.The surface and margins of each gland are greatly cut up, giving it the appearance of a somewhat branched white mass of gland.The duct of each gland begins to near its terminal anterior corner and immediately enters the muscles of the buccal mass and opens into the buccal cavity. The secretion of salivary glands contains mucin-like substance and a carbohydrate enzyme which digest starch.

3. Digestive gland:

The digestive gland (often referred to as the liver or hepatopancreas), which is a coiled and is of a brownish to dirty green colour and is quite soft and fresh. The digestive gland is pila is a somewhat triangular plate or cone with a very complex outer and more or less flattened inner surface . The cone is spirally coiled from the tip inwards and downwards following the whorls of the shell. It occupied the greater part of the coiled visceral masses.It has two main lobes, smaller in contact with the stomach and larger extending to the apex of the spiral.Two mail ducts arise from the main lobe of the digestive gland; these ducts unite just before reaching stomach to open into it by a common aperture.The digestive gland of pila: An apple Snail is made up of a number of fine tubules bound together by connective tissue. These tubules unite with one another to form larger tubules which terminate in two main ducts corresponding to two main lobes of the gland. The terminal part of each tubule is glandular, called the alveolus and the rest of tubule is ciliated.

Alveoli are lined with a digestive epithelium made up of three types of cells-

1. Sensory cells which secrets a cellulose-digesting enzyme,
2. Resorptive cells which digest proteins intracellularly,
3. A lime cell which stores calcium phosphate.

Semi-digested food enters into these alveoli, where digestion of cellulose and protein takes place.

Reference:

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L.,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

http://www.biologydiscussion.com/

https://en.wikipedia.org/wiki/Pila_(gastropod)