General Features:
External Features
Microscopic, elongated organisms. 80 to 350μ.
Oral groove is a shallow depression on the ventral part. Leads to the buccal cavity, which has a basal mouth called Cytostome.
Pellicle: Internal envelope of body. Clear, firm and elastic cuticular membrane. It consists of a series of polygonal depressions, with a single cilium emerging from the middle of each polygon.
Pellicle is a series of 3 membranes: Outer cell membrane, Outer alveolar membrane, inner alveolar membrane.
Cilia: Body is covered by numerous, tiny, hair-like projections called cilia.
Internal Features
Cytoplasm is differentiated into ectoplasm and endoplasm.
Ectoplasm has Trichocysts, oriented at right angles to the body surface. They alternate in position with the basal bodies of the cilia.
Contain a macronucleus and a micronucleus.
Macronucleus: polyploid, kidney shaped, much more DNA, controls metabolic activities (somatic nucleus), divides amitotically during fission.
Micronucleus: diploid, spherical, controls reproductive activities, divides mitotically during fission.
Two contractile vacuoles occupying somewhat fixed positions in the endoplasm (anterior and posterior).
Numerous non-contractile food vacuoles.
Cytopyge: Small portion of ectoplasm and pellicle is somewhat weak. During egestion, it functions as an anus.
Physiology
Movement through ciliary beats.
Holozoic mode of nutrition. Cilia of the oral groove generate a water current that is drawn in along with food particles into the food vacuole. The ciliary apparatus controls what particles can pass into the buccal cavity.
Respiration by diffusion through semi-permeable pellicle. Oxygen in water diffuses in and is used for oxidation of protoplasmic molecules. Catabolic wastes like CO2 and NH3 diffuse out.
Locomotion
Streamlined body which enables it to swim about in water with a minimum amount of friction.
Swimming is facilitated by Cilia – fine, hair-like cellular organelles that cover the entire animal’s body.
Energy needed for fibrillar contraction is supplied by ATP.
Ciliary Beats: During movement cilia oscillates, between fast effective stroke and slow recovery stroke. It propels the cilia similar to an oar. Recovery stroke brings the cilium into position for next effective stroke.
Metachronal Rhythm: All cilia of the body do not move simultaneously or independently. They beat progressively in a wave like pattern, beginning at the anterior end and progressing backwards.
A cilium in the longitudinal row always moves in advance of the one behind it.
All cilia in a transverse row beat simultaneously.
Mode of Swimming: Animal does not follow a straight tract but rotates spirally along a left handed helix. Body cilia beat obliquely towards right, so animal moves over to left on its long axis. Cilia of the oral groove strikes obliquely to turn the anterior end away from oral side. Combined effect causes movement of animal along a fairly straight path, rotating about its axis in an anticlockwise direction.
Conjugation
Not a true form of sexual reproduction, even though there is exchange of genetic material. It is simply a temporary union of two individuals which facilitates exchange of genetic material.
Process of conjugation:
Pre-conjugants of two different mating types come in contact at their oral grooves.
They stop feeding and their buccal structures disappear.
Pellicle and ectoplasm degenerate and at the point of contact, a protoplasmic bridge is formed between the conjugants.
Macronucleus disintegrates.
Micronucleus divides twice to form 4 micronuclei.
3 of the micronuclei disintegrate. The 1 remaining micronucleus divides to form 2 unequal gamete
nuclei. 1 gamete nuclei is male (motile and smaller) and the other is female (non motile and larger).
Motile gamete of one conjugant passes through the protoplasmic bridge and fuses with the non motile
gamete of the other forming zygote.
After formation of zygote, the conjugants separate and form ex-conjugants.
In each, zygote divides 3 times rapidly to form 8 nuclei. 4 enlarge to form macronuclei, other 4 are
micronuclei.
3 micronuclei degenerate and disappear.
The other micronuclei divides with mitotic division — 2 daughter paramecia from each ex-conjugant, each with 2 macronucleus, 1 micronucleus.
Another round of mitotic division produces 4 paramecia, each with 1 macro and 1 micro nucleus.
Conditions under which conjugation takes place:
Starvation and shortage of food.
Certain range of light and temperature, variable across species.
The conjugating individuals are usually smaller in size than the normal individuals.
Individuals are isogamous.
Only between individuals of same syngen but 2 different mating types (physiological sexual differentiation).
Significance of conjugation:
Rejuvenation: If binary fission continues for many generations, the Paramaecium loses vigour and enters depressed physiological efficiency. The individual ceases to multiply, reduces in size and eventually dies off. Conjugation can revive this lost vigour for asexual reproduction
Nuclear Reorganisation: During conjugation, there is readjustment between nucleus and cytoplasm. Replacement by a new macronucleus brings renewed vigour and vitality to metabolic functions.
Hereditary Variation: Blends genetic lines and introduces variations. Confers traits that can better adapt to environment.