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Paramecium caudatum from the decomposing leaf debris in my long-neglected freshwater garden pond. Imaged in Nomarski DIC on Olympus BH2 using SPlan 20x and 40x objectives plus variable phone camera cropping on Samsung Galaxy S9+. The population ranges from 187-225 um in length. The cells are cigar-shaped with the posterior end bluntly pointed and with a group of slightly longer cilia compared to the otherwise uniform ciliation. Early microscopists likened its shape to that of a slipper, and commonly referred to it as the "slipper animalcule." The oral groove/ vestibulum is long and slightly oblique; buccal cavity with one endoral membrane and 2 peniculi. There is a single large ellipsoid macronucleus and 1 round compact micronucleus. " Using light microscopy, however, it is possible to discriminate only the Mi type of P. caudatum from the nucleus of other two species. P. caudatum always has the Mi with the so-called “cap”, a space without chromatin at the nuclear pole" (1). Two contractile vacuoles, each with radial canals, are present at the aboral surface which serve to excrete excess water taken up from the outside, regulating the water contents of the body. Radially distributed "collecting canals" give the contractile vacuoles a distinctive star-like shape. The cell is enclosed by a cellular envelope (cortex) densely studded with spindle-shaped extrusomes called trichocysts.

Discussion adapted from: https://en.wikipedia.org/wiki/Paramecium_caudatum
P. caudatum feed on bacteria and small eukaryotic cells, such as yeast and flagellate algae. the accumulated food particles, at the posterior end of the cytopharynx, are directed by the long cilia into the rounded, ball-like mass in the endoplasm called food vacuole. The food vacuoles are circulated by the streaming movement of the endoplasm which is called cyclosis. In hypotonic conditions (freshwater), the cell absorbs water by osmosis. It regulates osmotic pressure with the help of bladder-like contractile vacuoles, gathering internal water through its star-shaped radial canals and expelling the excess through the plasma membrane. When moving through the water, they follow a spiral path while rotating on the long axis.

Paramecium have two types of nuclei, a large macronucleus and a varying numbers of smaller micronuclei depending on the species. P. caudatum has 1 compact round macronucleus. The macronucleus controls somatic gene expression and the micronucleus controls reproduction. Like all ciliates, Paramecia reproduce asexually, by binary fission. During reproduction, the macronucleus splits by a type of amitosis, and the micronuclei undergo mitosis. The cell then divides transversally, and each new cell obtains a copy of the micronucleus and the macronucleus. Fission may occur as part of the normal vegetative cell cycle. Under certain conditions, it may be preceded by self-fertilization (autogamy), or it may follow conugation, a sexual phenomenon in which Paramecia of compatible mating types fuse temporarily and exchange genetic material. During conjugation, the micronuclei of each conjugant divide by meiosis and the haploid gametes pass from one cell to the other. The gametes of each organism then fuse to form diploid micronuclei. The old macronuclei are destroyed, and new ones are developed from the new micronuclei. Without the rejuvenating effects of autogamy or conjugation a Paramecium ages and dies. Only opposite mating types, or genetically compatible organisms, can unite in conjugation.

I observed a Paramecium in the early mid stages of transverse fission with formation of two oral anlagen and a beginning constriction of the cortex dividing into two cells.

1. Paramecium genus: biodiversity, some morphological features and the key to the main morphospecies
   discrimination. Sergei I. Fokin. Protistology 6 (4), 227–235 (2010/11)
   https://www.researchgate.net/publication/285129115