and then, after a time, encysts to form a reproductive cyst. The encysted ciliate divides once, twice, or three times, and then excysts and begins feeding again. If the food supply is exhausted, resting cysts are formed. These are either temporary cysts or permanent cysts. Temporary cysts may excyst spontaneously, but permanent cysts require a definite stimulus and will not excyst for some time after encystment, during which time there is a period of reorganization.

Although this is the typical life cycle, division may take place without prior encystmeut (Penn, 1937, 368), and this condition may be experimentally induced (Stuart, Kidder and Griffin, 1939) Reynolds (1936, 49) found that when C. steinii is parasitic in snails it divides without forming a cyst. The life cycle of C. maupasi, which differs slightly from that of other species, has recently been described by Padnos et al (1951, 1953a, b). Conjugation within the family is extremely rare. Both Enriques (1908) and Wenyon (1926) recorded conjugation or syngamy in C. steinii, but I have observed conjugation of this species only once.

I have generally endorsed the accepted effects of pH, temperature and salinity on the active and encysted ciliates or have extended these observations. In one important point I differ in conclusion from those of C. V. Taylor et al. of Stanford University. Distilled water has proved perfectly effective for pxcystment without the addition of any exeysting factor (cf. Goodey, Bodine and Kiddar et al). Further, I have shown the particular significance of osmotic diffusion and the importance of osmotic difference between the milieux intericur and exterieur. The survival of Colpoda under anaerobic conditions, while agreeing with the ecological data, is in disagreement with Adolph's findings. These discrepancies may be attributed to differences in technique and the failure to allow gradual adaptation of the ciliates to changed conditions (Brand, 1946). The results are similar to those with Tetrahymena (Thomas, 1942) and Balantiophorus (Watson, 1944), in particular in the suppression of division.

The inhibition of division, and the consequent formation of what, has been called an unstable cyst, under the influence of adverse environmental conditions, is the chief new contribution to our knowledge of the life history of Colpoda. In this Colpoda resembles Woodruffia in which thin walled unstable cysts, larger than the permanent cysts, are formed as the result of extremes of temperature, high concentrations of H-ions, increased salinity, low oxygen tensions, crowding and feeding on starved Paramecium. In some respects the unstable cysts of Colpoda differ from those of Woodruffia. First, although both are large and thin walled and in this resemble the ordinary reproductive cyst, in Woodruffia there is r o record of the unstable cyst becoming a reproductive cyst upon the removal of the inhibiting factor, as is the general rule with Colpoda. Secondly, Colpoda is far more acid tolerant than Woodruffia and extreme pH does not cause unstable cyst formation. Colpoda continues to feed and live under low oxygen tensions, and only very rarely tonus unstable cysts. Woodruffia ceases feeding and encysts at low oxygen tensions and dies in the absence of oxygen. Woodruffia is carnivorous, whereas Colpoda is typically bacteriophagous. Crowding of active Colpoda is important in determining whether permanent or temporary resting cysts will be formed (Taylor and Strickland, 1938, 1939) but does not cause unstable cyst formation. Generally, then, there are close parallels between the two, but they are not altogether identical. It may be remarked that Johnson and Evans (1939, 95) described a third type of cyst, which they regard as a digestion

and assimilation cyst. In Colpoda, temporary cysts which excyst spontaneously are similar to normal resting cysts and are formed in the absence of food and the absence of crowding (Taylor and Strickland, 1939; Garnjobst, 1947). They are therefore different to the Woodruffia cyst. In Woodruffia the stable resting cyst requires an excystment medium similar to that allegedly required by Colpoda resting cysts, according to Taylor et al.

Tillina has apparently a simpler life history with only reproductive and resting cysts. The cysts are activated with distilled water. Bridgeman (1948) proposed that encystment was due to an insufficient amount of food, or an insufficient amount of oxygen, or both, and that it was accompanied by water loss. Excystment, she considered, occurred when food and oxygen levels are sufficiently high and there is an entrance of water into the cyst. In dealing with the resting cyst of the Colpodidae she is partially correct. However, she is in error when she considers the two processes—viz., encystment and excystment, as complementary, both under the influence primarily of food and oxygen. Absence of food, or nutritional deficiency, is an absolute condition of resting cyst formation in the Colpodidae, but it is not the only condition. Crowding, for Colpoda, is equally important, nor in this case is encystment affected by the oxygen tension. Indeed, encystment, in marked distinction to excystment, is independent of oxygen tension. Moreover, excystment both in Colpoda and Tillina can be achieved with distilled water. Excystment is then in these cases independent of food. Unlike encystment, however, it is dependent upon respiration, and is accompanied by a marked rise in the respiratory rate (Thimann and Commoner, 1940; von Wagtendonk and Taylor, 1942). The two processes then must be considered quite distinct and the factors affecting them quite distinct also. This distinction is shown in the following table:—

Another problem is the efficacy of potassium ions and alcohol as excysting factors. The protective function of the potassium ion in the alcohol-potassium excystment medium has also not been satisfactorily explained (Thimann and Haagen-Smit, 1937; Haagen-Smit and Thimann, 1938; Strickland and Haagen-Smit, 1947, 1948). This is partly due to the fact that those who worked on these factors were unaware that exeystment of Colpoda can be achieved simply with distilled water. Such substances as hay extract, organic acids and so on have not, therefore, the role of essential metabolites. In this case Bridgeman is also misleading, for alcohol can hardly be considered as food. She is on safer ground when she stressed the fundamental matter of osmotic differential. It has been shown (Bodine, 1923) that alcohol in low concentrations rapidly penetrates the cyst membrane. It may be presumed that metabolites such as the organic acids readily diffuse through the cyst membrane as shown by Bodine (1923) and Haagen-Smit and Strickland (1938). The particular efficacy of the potassium ions, found by these last authors, may therefore be attributed to their ready diffusion through the membrane and consequently their stimulus to further osmosis. Once osmosis is established, for example with distilled water, the excystment process is largely, though not wholly automatic, provided oxygen is present