Pila Globosa – The Apple Snail


EXCRETORY SYSTEM

Excretory organ of molluscs is called Organ of Bojanus or kidney. In Pila it is made of an anterior and a posterior renal chamber which extract wastes from the blood. The posterior renal chamber also collects wastes from the pericardial cavity through a renal duct. The lining of the pericardium carries podocytes which secrete wastes into the pericardium.

The posterior chamber is connected to the anterior chamber through an opening. The anterior chamber is reddish in colour and has leaf-like lamellae for extracting nitrogenous wastes. Opening of the anterior renal chamber which is also called the excretory pore lies in the branchial chamber near the right nuchal opening. The excretory products in Pila are ammonia, urea and uric acid. Digestive gland also performs some excretory function in apple snail.

NERVOUS SYSTEM

The nervous system is asymmetrical called streptoneury, owing to the torsion of visceral mass. A pair of cerebral ganglia on the dorsal side in buccal mass is connected by the cerebral commissure. A pair of buccalganglia is located in the floor of the buccal cavity. On the ventral side of the buccal mass is a pair of pleura-pedal ganglia that are connected to the cerebral ganglia by cerebro-pleural and cerebro-pedal connectives. Pedalganglia are connected together by the pedal commissure. Statocysts are located in the foot and are connected to the pedal ganglia. This part of the nervous system located in the buccal mass is symmetrical but the posterior part is twisted in a figure of English alphabet 8. Visceral ganglion lies in the posterior part of the visceral mass.

The pleura-pedal ganglion is connected to the visceral ganglion by the infra-intestinal-visceral connective on the right side. On the left side, the supra-intestinal ganglion is connected to the visceral ganglion by a supra-intestinal-visceral connective and on the anterior side this ganglion is connected to the pleura-pedal ganglion by zygoneury connective. Supraintestinalnerve travels above the intestine and connects the supraintestinal ganglion with the right pleuropedal ganglion. The infraintestinal nerve travels below the intestine and connects both sides of pleural ganglia. Most parts of the nervous system are concentrated on the anterior side.

SENSE ORGANS

Eyes, tentacles, osphradium and statocysts are the chief sense organs of the apple snail. Mantle is also tactile in function.

Osphradium is attached to the mantle and is made of a central axis and 22-28 fleshy leaflets that carry chemoreceptors.

Statocyst is a pair of organs of balance and posture located in the foot. It is a hollow ball-like structure surrounded by connective tissue and whose epithelium carries sensory cells that are connected to the nerve. Inside the hollow ball there are gelatinous fluid and calcareous statoconia. When the animal moves these statoconia touch the hairs of sensory cells and the nerve impulse goes to brain giving the kinetic sense to the animal.

Eye is simple whose outer cornea is made of epidermis of the skin. Inner cornea is made of the epithelium of the optic capsule, which also houses an oval crystalline lens. The sensory retinal cells and pigmented cells surround the lens and they are also connected to the optic nerve that carries the impulses to the brain. This is simple eye connected to the head by a stalk or ommatophore and found in most of the molluscs.

REPRODUCTIVE SYSTEM

Sexes are separate. Testis is whitish located in the coils of intestine and closely attached to the digestive gland. Vasa efferentia join to form a vas deferens that terminates into a vesiculaseminis where maturation of sperms takes place and seminal fluid is added. The glandular part of vas deferens is wider and carries hypobranchial glands for lubrication of the passage. There is a small penis for introducing the sperms into the reproductive tract of the female.

Ovary is branched and orange in colour and located in the coils of intestine near the digestive glands. Oviduct is transparent and terminates in a deep yellow coloured uterus. There is a bean-shaped seminalreceptacle that stores sperms from male after copulation. Vagina is long and terminates in female genital opening near which there is a rudimentary penis. As the ova come down the oviduct, they are fertilized by the sperms derived from the seminal receptacle.

Female deposits 200-800 eggs in moist soil where they hatch into tiny snail. In marine gastropods there is a veliger larva that hatches from eggs and then metamorphoses into adult. In freshwater and terrestrial gastropods both the trochophore and veliger stages are passed inside the eggs and tiny snail hatches from the eggs. Torsion takes place within 15-30 minutes after hatching from the eggs after which the growth and shell formation takes place.

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