Systematic Position of Paramoecium
Structure of Paramoecium
Locomotion in Paramoecium
As paramecium is a ciliate animal so it moves by its cilia. The rapid swimming is facilitated by the beating of fine and hair-like cellular organelles, called cilia, that cover the animal’s entire cell-body. Paramoecium moves with a speed of 1500 µm or more per second.
Structure of Cilia under light microscope-
Cilia are the principal locomotory organs of Paramoecium. Structurally, cilia and flagella are similar but they differ in their arrangements and number. Flagella are one, two or a few having localized arrangement but cilia are arranged in rows. Flagella are longer but cilia are shorter. In Paramoecium cilia are arranged in longitudinal rows. The entire body is covered with cilia so it is known as Holotrichus.
Structure of Cilia under light microscope-
The microfibrils forming the basic skeleton of a cilium together known as Kinetodesmata. Each kinetodesma resides within the cytoplasm and directly areises from a basal granule Cilia are arranged in a row and each row is known as kinety. Each cilium shows a complicated arrangement of basal granule and kinetodesma. In a kinetodesma the number of fibrils are may exceed 500. Besides these there are contractile microfibrils below the pellicle of Paramoecium. They may join to basal granules.
On the basis of the structure each cilium of a Paramoecium is divided into two regions. They are-
(i) Kinetodesmal region and (ii) Shaft region.
Each kinetodesmal region shows (9+2) fibrillar arrangement like a flagellum, i.e., there are nine peripheral and fibrils and two central fibrils. Each cilium originates by a double root, roots join to basal granules from which kinetodesmata arise. Each cilium is bounded by a unit membrane and it is continuous with the plasma membrane. Each outer fibril consists of two sub-fibrils, sub-fibril A and sub-fibril B. The central two fibrils have no sub-fibrils and remain bounded by a common sheath. According to Gibbons (1967) the sheath of the central fibrils gives out nine radially arranged spoke like structure which joins with the sub-fibril A. The inner arm of subfibril A joins with the sub-fibril B of the next fibril in a clock-wise direction. The basal bodies, kinetodesmata and tubules of the cilia together constitute the infraciliature of ciliated animal like Paramoecium.
Process of ciliary locomotion
Like flagellate protozoa ciliate protozoa also live in water and therefore, their movements are caused by striking the water with cilia. These strokes are irregular and contain only one wave. Each cilium has an effective stroke and a recovery stroke. In the recovery stroke there is no movement. As cilia are many but shorter in length so there is a coordination in their movement and this coordinated movement is known as metachronal movement.
During an effective stroke the protein molecules of the central fibrils contract and the cilia bend to strike the water. This is the effective stroke which creates sufficient force to drive the animal forward. After the effective stroke is completed cilia tend to return to its original position generating minimum force by alternate events of that helped in effective stroke. This is recovery stroke. Effective strokes and recovery strokes are the main principles of ciliary locomotion. Some cilia of the kinety perform effective stroke whereas some recovery stroke. In this way a wave of metachronal rhythm pass from anterior to posterior region and the animal tend to move forward. As the oral cilia are larger and they beat more quickly thus helping the animal to rotate on its axis and the animal moves forward while rotating on its own axis.
Molecular mechanism of Ciliary Movement-
To explain the mechanism of ciliary movement at the molecular level two theories have been proposed. They are-
- The localized contraction theory- According to this theory contractile units are placed at regular intervals along the length of the axoneme. The contraction of these units help to bend the cilia. During ciliary movement there is a change in the length of the subfibrils of a doublet. Molecular contraction and relaxation would explain both the effective and recovery stroke. During effective stroke one set undergoes contraction while other set streatches and during the recovery stroke the reverse would take place.
- The sliding filament model- This theory states that bending of the cilium is initiated by sliding of the tubules of the peripheral fibrils relative to one another. The two subfibres A and B of a doublet do not move relative to one another because they share a common wall although the termination distance of the subfibres remain constant during the effective and recovery strokes. So, it is supposed that the filaments must be moving against adjacent doublet and thus effects a ciliary movement.