Pila Globosa – The Apple Snail


Pila globosa is an amphibious apple snail found in the Indian subcontinent. Its flesh is commonly used as prawn feed in aquaculture. It is also used as poultry feed and also as human food in some places. This species uses water bodies, rice fields and highly humid areas for breeding.

Pila shell is unilocular in which all the whorls are internally connected together. The single opening of the shell is called peristome which is closed by a hard operculum. Body that is covered by mantle consists of head, foot and visceral mass.

An epitaenialridge divides the mantle cavity into a pulmonary and a branchial chamber as Pila is an amphibious animal. Water enters the branchial chamber through the left nuchal opening and goes out through the right nuchal opening. Branchial chamber contains a single monopectinate gill or Ctenidium. The openings of the rectum, gonoducts and excretory ducts also open in the branchial chamber at the base of right nuchal lobe so that their products are expelled along with the outgoing current of water.

Operculum is opened to make the foot extend out of the shell for locomotion. Head bears a pair of stalked eyes and two pairs of tentacles. First pair of tentacles is also called labial palp as it helps in feeding.

DIGESTIVE SYSTEM

Pila is herbivore and has a pair of strong jaws for cutting and chewing. It also has a belt-like radula with teeth to corrode and rasp algae from the surface of rocks. A pair of whitish buccalglands secrete mucous in the buccal cavity. Oesophagus is long and passes through the buccal mass or odontophore, which is a heavy and large structure containing cartilages, protractor and retractor muscles of radula and odontophore muscles and tissues. Oesophagus also contains a pair of salivaryglands located inside the oesophageal pouches. These release saliva that carries starch digesting enzymes.

Stomach is large sac like divided into cardiac and pyloric portions with a pyloric caecum that carries cellulose digesting bacteria. Intestine is long coiled inside the visceral mass and leads to rectum that opens by anus near the right nuchal opening. A large brownish to greenish digestivegland, which has a larger and a smaller lobe, is present in the coils of intestine and opens into the pyloric stomach by a duct. Digestive enzymes are secreted by this gland into the stomach. Gonads are also present in the coils of intestine in the visceral mass. Intracellular digestion also takes place inside the digestive gland by amoeboid cells.

RESPIRATORY SYSTEM

Pila is an amphibious animal and carries a single monopectinate ctenidium on the right side of the branchial chamber, separated by the epitaenidial ridge from the pulmonary chamber. Water current enters through the left nuchal opening and passes out through the right nuchal opening. An osphradium, which is a chemoreceptor, is located near the left nuchal opening. 

For aerial respiration respiratory siphon of the pulmonary sac is thrust out of the left nuchal opening to inhale outside air into the single pulmonarysac, which is supplied with network of blood vessels. Inhalation and exhalation is carried out by the muscles that surround the pulmonary sac. During aerial respiration the epitaenialridge shuts off the branchial chamber so that it does not get dried up by air.

CIRCULATORY SYSTEM

Pila has an open type of circulatory system that consists of blood vessels, lacunae and sinuses. Heart is made of an auricle and a ventricle, the latter gives off a cephalic aorta and a visceral aorta. Cephalicaorta supplies blood to the osphradium, foot, pulmonary sac and the mantle. The visceralaorta supplies blood to the liver, kidney and other visceral organs including the digestive system. From the kidneys the blood is collected by renal vein and taken to auricle. Also from the kidneys afferentctenidial vein takes the blood to gills for oxygenation. From gills the efferent ctenidial vein collects blood and carries it to the auricle of heart. From the pulmonary chamber the blood is collected by the efferentpulmonaryvein and taken to the auricle.

There are numerous small lacunae in which the blood vessels end. All lacunae open into 5 major sinuses, namely, perivisceral sinus, peri-intestinal sinus, branchio-renal sinus, pulmonary sinus and the pericardial sinus.

Respiratory pigment in the blood is haemocyanin dissolved in plasma and there are blood corpuscles such as amoebocytes.

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