The gullet and trichocysts of Cyathomonas truncata

Evidence is presented for the passage of bacterial food organisms into a cytoplasmic tube present in Cyathomonas truncata . The tube thus appears to function as a gullet or cytostome. A fringe, which may assist in feeding, surrounds the entrance to the gullet. Microtrichocysts and macrotrichocysts are recognized in this organism. The undischarged trichocysts bear a strong resemblance to retractile bodies of kappa particles, and are able to incorporate 3 H-thymidine. The trichocysts of Cyathomonas can thus be classified with that ever increasing category of cytoplasmic organelles whose ultrastructural and molecular properties suggest that they are a type of endosymbiont.