PARAMECIUM

Habit and Habitat

                             

Paramecium, commonly known as slipper animalcule due to its look like a sole of the slipper,is found in fresh water pond, pools,rivers,streams,etc. It is abundantly found in the stagnant water which is rich in decaying organic material.

Cell Structure

Paramecium has an elongated, asymmetrical,spindle shaped body measuring about 80 to 350 micron in length. The anterior end of the body is blunt and the posterior end is pointed. The shape of Paramecium looks like a sole of the slipper. The structure of Paramecium is comparatively much complex due the formation of various organelles.

1. Pellicle. The body is covered by thin, firm and elastic, protective covering called pellicle. It provides a definite shape to the body of the animal. being elastic, it also help in contraction of the body. the pellicle is distinguished into innumerable hexagonal or polygonal facets which has a minute aperture in the central through which a cilium. project out. The margins of the hexagonal facets bear the opening of trichocysts
2. Oral groove and associated structure. A large, oblique and shallow oral groove (or peristome ) is present on the ventro-lateral surface that leads into a tunnel shaped structure called vestibule which opens behind into tubular  buccal cavity. Below the buccal cavity is a small non-ciliated structure called cytopharynx. A small mouth (cytostome) is present at the junction of buccal cavity and cytopharynx.
3. Cytopyge (Anal spot). A minute aperture called cytopyge on the ventro-lateral surface of the body behind the cytostome occurs whose function is to expel the undigested food out of the body.
4. Ciliation. Arranged in parallel rows all over the body, innumerable cilia are present. They are locomotory and food catching organelles of Paramecium.Cilium arises from a basal granule (kinetosome) and they are of equal size except for a few at the  posterior end which are long and constitute the caudal tuft. All the cilia of the body are divided into two type i.e.(1) Somatic cilia, present all over the body, (2) Oral cilia present in the buccal region. They are specialized, hard and long cilia and help in food ingestion.

Internal Structure 

1. Cytoplasm. The cytoplasm of Paramecium is differentiated into two layers,(1) Outer ectoplasm (cortex) . and (2) Inner endoplasm(medulla). The ectoplasm is clear and dense part of the cytoplasm which forms its outer layer below the pellicle. Numerous ectoplasmic inclusions include the basal granules, neuromotor system and trichocysts. Each basal granule represents a kinetic energy centre.
2. Neuromotor Apparatus. All the basal granules of a row are connected with each other by longitudinal fibres called myonemes. They converge a bilobed body called motorium which is situated near the constitute the neuromotor apparatus provides conductivity and elasticity to the body.
3. Trichocysts. The trichocysts are minute spindle-shaped structure which open on the surface of the pallicle by means of a minute aperture placed on its hexagonal ridge.They are the organelles of defence and offence. The inner part of the cytoplasm ,the endoplasm is thin, semiliquid and granular. Many structure such as nuclear apparatus, contractile vacuoles and food vacuoles are suspended in it.  
4. Nuclear Apparatus. The nuclear apparatus comprises the two nuclei of which one macronucleus which is large kidney-shaped and granular and contains trophochromatin and controls the metabolic activities of the body and divides automatically at the time of cell division. The other nucleus known as micronucleus which contains idochromatin is small round and more compact and has a definite nuclear membrane. It control the reproductive processes and  divides by reduction division at the time of cell division.
5. Contractile Vacuole. There are two large contractile vacuoles one of which is situated near the anterior end while the other near the posterior end. These are nearly spherical, extremely elastic and water-filled structure. Each of the contractile vacuole is surrounded by 6 to 10 narrow spindle shaped radiating canal. The contractile vacuoles are osmoregulatory in function. They collect the excess water from the cytoplasm and remove it to the outside.The contractile vacuoles also act as excretory organelles. Many food vacuoles of different size and shape are found in the endoplasm. Each vacuole encloses a few food particles along with a drop of water.