ABNORMALITY IN THE FISSIONING CONTROL OF A STRAIN OF ASEXUAL DUGESIA-DOROTOCEPHALA FOUND NEAR FORT-COLLINS, COLORADO

Asexual Dugesia dorotocephala, collected from a new site several miles northwest of Fort Collins, Colorado, appeared similar, at the time of collection, to ones collected from other sites in the same general lo cale. However, these exhibited marked differences in the more crowded conditions of laboratory culture: abnormally long, 30-50 mm specimens, seemingly unable to fission, developed. Since previous studies of ase xual D. dorotocephala had revealed an inhibitory effect of cohorts on fissioning (Best et al., 1969, 1974, 1975; Pigon et al., 1974), experi ments were conducted with this new strain to ascertain the effect of p opulation density (group size) on fissioning incidence. Results are de scribed which show that this inhibitory effect of cohorts on fissionin g is exaggerated in this new strain.