The Fine Structure of the Phoront of the Apostomatous Ciliate,Hyalophysa chattoni*

The phoront of the apostomatous ciliate, Hyalophysa chattoni, is an encysted stage that is carried on the exoskeleton of its crustacean host until the ecdysis of the host. At molting the phoront rapidly metamorphoses to the feeding stage, excysts, and immediately begins to feed on exuvial fluid trapped in the cast-off exoskeleton. The fine structure of the resting phoront resembles that of the preceding migratory stage, the tomite. A prominent ventral tuft of cilia, the ogival field, has vanished, and the trichocysts that paralleled the kinetics have disappeared. The dense inclusion bodies that were concentrated around the mouth and falciform fields have dispersed and greatly decreased in number. The cytoplasm and its membranous organelles do not appear visibly condensed or altered from the preceding stage in the life cycle. The phoront is merely quiescent instead of dormant. Unlike the few ciliate cysts previously examined by electron microscopy, the phoront's cyst is not divisible into separable layers. It resembles the loricae of certain suctoria in being formed principally of a fibrous substance, the outer surface of which has a paracrystalline pattern. The peduncle attaching the cyst to the crab's gill is a continuation of the cyst wall although its structure is somewhat modified. The most conspicuous innovation in the phoront's fine structure is the massive tracts of microtubules that run longitudinally through the macronucleus. The microtubules are in intimate contact with Feulgen-positive chromatin masses which are crowded toward the periphery of the macronucleus.