Ultrastructural features of Allantosoma intestinalis, a Suctorian ciliate isolated from the large intestine of the horse.

Allantosoma intestinalis, a suctorian ciliate isolated from the intestine of the horse, was studied utilizing light and electron optical methods. These small sausage-shaped organisms have a varying number of tentacles (between one and 14) located at each extremity of the body. The microtubular axoneme of each tentacle in cross-section consists of two files of microtubules arranged in a daisy-like configuration. Haptocysts occur in the tentacle shaft, abutted to the plasma membrane of the knob of the tentacle, and in the cell body. The haptocysts are bottle-shaped, with prominent annular striations around their midportion. The cell is covered by three membranes, an outer plasma membrane, an outer alveolar, and an inner alveolar membrane. A thin epiplasmic layer is found beneath the inner alveolar membrane, and a single row of microtubules underlies the epiplasm. The subpellicular microtubules are arranged parallel to each other forming a corset around the cell along the long axis; such a system is not characteristic of suctorians. A field of diminutive kinetosomes (each 180 mm long, max, of 15 per field), lacking cilia, was found below the cortex. The function of these prokinetosomes is unknown. A ciliated swarmer has not been observed, only the nonciliated adult. The characteristics of Allantosoma are compared with those of other suctorian genera.