Oral Groove The ventral surface of paramecium shows a large shallow oblique depression, called oral groove, which extends from the middle to the left side of the anterior end of the body. Posteriorly, the oral groove leads into a deeper conical vestibule, which in turn communicates with a buccal cavity having a basal mouth called cytostome. Pellicle The body of paramecium is bounded by a thin, firm, elastic and colorless cuticle membrane, the pellicle or periplast. It is secreted by the underlying ectoplasm. The pellicle helps paramecium to have a definite and constant body shape. The flexible, elastic and contractile nature of pellicle allows metaboly i.e., distortion of body form while squeezing through cervices. The pellicle, in paramecium, is not a smooth layer. If the animal is placed in 35% Alcohol or seen untreated under electron microscope(EM), its surface seems to be divided into a great number of very small hexagonal areas or ciliary fields formed by the crossing of obliquity running ridges bearing the openings of the Trichocycts. Each hexagonal plate has a central depression called cirum- ciliary space perforated by a pore, through which a single flagellum projects out.