Temporal suspension of excystment by salt solution in the ciliate, Histriculus cavicola (Kahl, 1935) (Ciliophora: Stichotrichia).

Cysts were incubated in excystment medium (EM) for varying periods and then transferred to salt solutions or to 5 μg/ml cycloheximide (CH). When the transfer was to salt solutions, almost no cell excysted within 24 h after initiation of incubation in EM if the incubation was shorter than 30 min, but the proportion of excysted cells increased with increasing incubation time longer than 40 min. When the transfer was to CH, no cell attained stage 2 of excystment after incubation in EM shorter than 30 min but normal excystment ensued after an incubation longer than 70 min. If incubation in EM was between the above mentioned periods, some cells attained stage 3 but never excysted. Kinetosome formation occurred after incubating cysts for 30 min in EM, regardless of the presence or absence of CH. Formed kinetosomes, however, disappeared if cysts were maintained in salt solutions for 210 min or in CH for 240 min. These results indicate that: 1) excystment requires protein synthesis and incubation of cysts in EM longer than 30 min, 2) kinetosome formation does not need protein synthesis and does not necessarily direct cysts to complete excystment, and 3) maintenance of new kinetosomes requires continuation of the excystment process.