Important external structures of paramecium are listed below:
(i) Size:
Paramecium is a microscopic organism. Its size varies in length from 80 µ to 350 µ caudatum is the largest species measures between 170 µ and 290 µ.
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(ii) Shape:
Paramecium is often described as slipper shaped, cigar-shaped or spindle-shaped. Its shape is usually constant and is generally asymmetrical.
The body is elongated, blunt and rounded at the anterior end and somewhat pointed at the posterior end. In cross section, it is circular with greatest diameter little behind the centre of body.
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The anterior half of body is slightly twisted. The body is distinguished into an oral or ventral surface and aboral or dorsal surface.
(iii) Oral groove:
Ventral surface of body bears a prominent, oblique and shallow depression called oral groove. It originates from the middle of body and extends to the left side of anterior end.
Posteriorly, the oral groove leads into a deeper conical vestibule which, in turn, communicates with a buccal cavity having a basal mouth or cytostome.
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(iv) Pellicle:
Body is covered by a living, clear firm and elastic cuticular membrane, the pellicle. Electron microscopic studies by Ehret and Powers (1959) have revealed that pellicle is composed of regular series of cavities, the alveoli.
All the alveoli collectively form a continuous alveolar layer. Alveolar layer is delimited by an outer alveolar and inner alveolar membrane. The outer alveolar layer lies in close contact beneath the outer cell membrane.
(v) Cilia:
The entire body surface is covered by numerous, tiny, hair-like fine projections, called cilia. These measure 10-12µ in length and 0.27 µ in diameter.
Each cilium arises from the centre of each alveolus of the pellicle. Cilia are arranged in regular longitudinal rows.
Their length is uniform throughout, except for a few longer cilia at the extreme posterior end of the body forming a caudal tuft, hence the species name caudatum.