Respiratory system of Pila: An apple snail.

Respiratory System:

Pila show both aquatic as well as aerial modes of respiration: The respiratory organ o0f pila consists of a single ctenidium or gill, a pulmonary sac or lung and a pair of niches lobes. Left and right nuchal lobes act as accessory structures.

[I] Respiratory organs:

1. Ctenidium.

It is a comb-like mono pectinate gills and situated on the right side of the branchial chamber hanging from its dorsolateral wall.It is composed of a long series of thin triangular leaflets or lamellae, lying parallel to each other, which are attached to the mantle wall by their broad bases but have their apicals hanging free in the branchial chamber.The line of attachment of the lamella to the wall of the mantle forms the ctenidal axis.Ctenidium axis is transversed by an afferent blood vessel and an efferent blood vessel. Afferent blood vessel carries deoxygenated blood to the gills and the efferent blood vessel carries oxygenated blood from the gills to the heart. Axis bearing a long serious of flat triangular leaflets known as lamella. All the gill lamella are not of the same size; they are largest middle and gradually smaller towards the ends.Such type of gill is known as a mono pectinated gill. Each lamella bears a transverse ridges or plates on both its anterior and posterior surfaces.These 0plates are low ridges gradually decreasing in size contain branches of blood vessels , through which the blood flows from the afferent blood vessel to the efferent blood vessel.

Ctenidium is innervated by nerve from the left pleural and supra-intestinal ganglia. This nerve supply indicates that it is actually an organ of the left side but has shifted to the right during development. Histologically each lamella is formed of a double layer of epithelium which encloses a narrow cavity. Each epithelium layer consists of

1. non-ciliated columnar cells.

2. ciliated columnar cells, and glandular cells.

At the base of the layer are connective tissue cells and oblique muscle fibres.

2. Pulmonary sac:

The pulmonary sac or lung is a closed cavity bag like structure which is developed f from the mantle wall.It hangs from the dorsal wall of the mantle cavity into which it opens through a large, oblique pulmonary aperture. Dorsal surface of the pulmonary chamber is pigmented, while the ventral wall is a creamy wall. The wall of the sac are muscular and highly vascular, i,e, richly supplied with the blood vessel.

3. Nuchal lobes:

The right and the left pseudopodia or nuchal lobes are highly contractile and fleshly structure of the mantle; situated one on either side of the head. These two nuchal lobes form a sort of drain for the entry and exit of water in and out of the mantle cavity.

[II] Mechanism of respiration:

Mechanism of respiration takes place by Aquatic and Aerial respiration.

(i) Aquatic respiration:

Aquatic respiration takes place when the snail is at the bottom of a pond or aquarium,w hen it is floating or lying suspended in mid- water or when it is attached to plants or weeds in the water. At this time the animal is found to be fully expanded with the head and foot fully extended. The two nuchal lobes further increase in size and the left lobe takes the form of a distinct gutter in which a current of water flows. In aquatic respiration, the water current enters the left nuchal lobe and first come in contact with osphradium which tests the nature of water. It enters the mantle cavity and passes over the epithelia into the branchial chamber to bathe endothelial layer blood space the ctenidium, then the current passes out through the right nuchal lobe.The flow of water current is maintained by

1. the alternate lowering and raising of the floor of the mantle cavity; this is brought about by alternate protrusion and retraction of the head, and

2. by the beating of cilia on the lamella of the ctenidium.

The ctenidium takes in oxygen from the current of water and gives out carbon dioxide which diffuses into water.

(ii) Aerial respiration:

This mode of respiration is observed when the animal is in water or out of the land. The pulmonary sac or lungs are used in ariel respiration in two ways. When the snail comes to the surface of water its left nuchal lobe projects as a tube above the water and the air is drawn into it, the air goes to the pulmonary chamber and then into the lung; the branchial chamber being shut off by the epithelia pressing against the mantle.When the snail comes on land it takes air directly into its lung through the mantle cavity and no syphon is formed by the left nuchal lobe.In both the aerial respiration alternate contractions and relaxations of the muscle of the lung wall take place, when the muscle contract the floor of the lung gets arched increasing its cavity of the lung decreasing and air is expelled, inward and outward movement of the head and foot also help in the process of taking in air. An apple snail also respires by its pulmonary sac during aestivation period by means of the air already imprisoned in the air. The blood vessel in the lung take in oxygen from the air and give out carbon dioxide.Respiration takes place with that much of air which is stored in the pulmonary sac.

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)