The Life Cycle and Morphology of the Apostomatous Ciliate,Hyalophysa chattoni n. g., n. sp.*

SYNOPSIS. A new apostome ciliate was discovered in collections at Friday Harbor, Washington and the San Francisco area. All stages of the life cycle were studied in both living and stained condition. Dormant encysted stages (phoronts) occur on the gills of Pagurus hirsutiusculus. Excystation occurs in synchrony with the molting of the host yielding the trophic stage (trophont), which feeds on the exuvial fluids trapped in the crab's cast-off exoskeleton. The trophont becomes greatly enlarged as a result of feeding, and the cytoplasm and organelles become compressed into a thin cortical layer. Each fully grown trophont encysts (becoming a tomont) as a prelude to repeated binary fission, which results in the release of actively motile offspring (tomites). These disperse and promptly resume the encysted phoront stage on the host's gills. The Chatton-Lwoff silver impregnation method revealed that all stages of the life cycle have nine somatic kineties. In the trophont stage they are accompanied by an anterior ventral field of scattered clumps of kinetosomes. During conjugation the partners attach by their ventral surfaces between kineties 1 and 9 and at the left of the ventral field. The tomite stage was stained with Protargol. In addition to the characteristic features of the foettingeriid tomite also revealed by the Chatton-Lwoff method, Protargol revealed the following heretofore undescribed morphological features: a short row of kinetosomes immediately anterior to the ogival field; a line paralleling the left margin of the field; the continuity of kinety 8 with falciform field 8; the entrance of kinety 9 into the mouth and its ending against the rosette (an enigmatic organelle characteristic of Foettingeriinae). Feulgen stains showed that the chromatin in the macronucleus is dispersed in aggregates whose size and number vary with the stage of the life cycle. The major period of chromatin synthesis appears to be during the early tomont stage, when Feulgen-positive material increases visibly in amount and intensity of staining. This apostome ciliate was characterized as a new genus on the basis of the infraciliature of the trophont stage, its conjugation with ventral surfaces appressed, and its life cycle. It is named Hyalophysa (hyalo = glassy, physa = bubble) chattoni.