A Comparison of the Parasitic Phase of Tetrahymena limacis (Warren) with Clones in Culture, with Particular Reference to Variability in the Number of Primary Ciliary Meridians

SUMMARY. The number of complete primary ciliary meridians in Tetrahymena limacis found in natural infections in the slug Deroceras reticulatum has been observed to range from 32 to 40. To study the range in the number of meridians in clones in culture, 10 ciliates were pipetted singly from each of two natural infections into sterile 1% (w/v) yeast extract to which 400 units of potassium penicillin‐G and 500 micrograms of dihydrostreptomycin sulfate per ml. had been added aseptically. Ten ciliates were also pipetted from each of these natural infections into infusions of tissue of the digestive gland of D. reticulatum. Altogether, 28 clones (13 axenic clones in yeast extract and 15 bacterized clones in tissue infusions) were established. The shape of ciliates in thriving axenic cultures is generally ovoid or pyriform, and similar to that of ciliates in bacterized cultures. However, ciliates in axenic cultures are sharply apiculate at the anterior end and free from inclusions other than small lipoid granules, and in these respects resemble ciliates of the parasitic phase. The cytostome of ciliates in bacterized and axenic cultures is larger in proportion to the size of the body than it is in the parasitic phase. The ranges in the number of ciliary meridians in ciliates of the two natural infections from which the clones were established were 33 to 40 and 32 to 38, whereas the number of meridians in the clones taken as a whole ranged from 25 to 32. Fifteen of the clones showed extremes of 26 and 30. One of the clones was re‐introduced into D. reticulatum by exposing 50 slugs to a culture of the ciliates for 10 days. The slugs were then transferred to clean containers for another 10 days. Forty‐one of the slugs exposed to ciliates survived the experimental period, and 32 became infected by T. limacis. Forty‐four of 50 slugs in an unexposed control group survived, and none were infected by ciliates. In the ciliates from slugs of the experimental series, the number of complete primary meridians ranged from 28 to 36. However, more than half of the ciliates in the experimental infections possessed incomplete rows, and some individuals had as many as five of these. Ciliates in natural infections and in cultures sometimes have incomplete rows, but rarely show more than one such row. The incomplete rows in ciliates re‐introduced into slugs are considered largely to be incipient segments of future complete rows. They occur most commonly in the post‐oral region, and particularly between row 1 (the so‐called stomatogenous meridian) and the next complete row to the observer's right. It appears, therefore, that the post‐oral region is a zone in which most new rows are proliferated.