The life history of Vorticella microstoma is shown diagrammatically in Text-fig: 1. There is both a sexual and a non-sexual cycle, and the nonsexual cycle normally includes a division and a cystogenic cycle.

In the non-sexual cycle the stalked trophic ciliate, with its peristome everted, feeds and increases in size Division takes place by longitudinal fission, which is slightly asymmetric so that one daughter cell remains attached to the parent stalk. The other forms a posterior ciliary wreath and with its peristome retracted becomes a free-swimming non-trophic telotroch. The polarity of the telotroch is the exact reverse of the stalked individual, the ‘posterior ciliary wreath’ of the settled form now being foremost in locomotion. The telotroch rotates in a counter-clockwise direction. Ultimately it settles, secretes a stalk, resorbs the posterior ciliary wreath, everts the peristome and begins feeding as a normal trophic individual. On the exhaustion of food, and sometimes under the influence of other environmental factors, the peristome is retracted and the ciliate secretes a cyst membrane (Brand, 1923). When stimulated to excyst, the ciliate escapes from the cyst membrane as a telotroch and then reverts to the normal trophic cycle (Stout, 1954).

Under certain circumstances conjugation takes place (Finley, 1936, 1939) Preconjugation division leads to the formation of a macrogamete and a microgamete, the latter very much smaller but otherwise morphologically identical with the telotroch. The free-swimming microgamete becomes fused with a macrogamete and conjugation takes place, the microgamete being completely absorbed. The microgamete can conjugate with its sister macrogamete but not with an ordinary trophic vorticellid which the macrogamete otherwise resembles. Unlike the telotroch it dies if it fails to conjugate. It cannot metamorphose into a normal tropaic individual, although the macrogamete can. After conjugation the vorticellid reverts to the non-sexual cycle.

Although the normal vorticellid has two stages, one sessile and trophic and the other non-trophic and free-swimming, occasionally the stalked individual becomes detached from its base. When this happens the individual will swim freely in the medium propelled by the beat of its peristomial cilia which normally cause a current of food particles to be swept towards the vestibule.

One change not mentioned in the above account is the habit of direct telotroch formation by the trophic vorticellid. This is a characteristic response to a number of environmental factors of which low oxygen tension and high carbon dioxide tension are two demonstrated in the present paper. Previously it has been held (Brand, 1923) that the vorticellid encysts in response to “adverse” conditions but this is rarely the case, except in response to starvation. The most common response is the formation of a free-swimming telotroch which is able to survive such conditions as lack of oxygen and high carbon dioxide tension for a comparatively long time.

Text-fig 1.—Diagram of the life history of Vorticella microstoma. The sexual cycle has been described by Finley (1939) and cystment and division by Brand (1923) and Stout (1954). The present studies are concerned with excystment and the direct metamorphosis of the trophic to the telotroch form (shown by the broken line) in response to exceptional environmental conditions. It is apparent that this metamorphosis interrupts all three normal cycles, namely the sexual cycle, encystment and division.