OLD AND NEW DATA ON SEISONIDEA (ROTIFERA)

Class Seisonidea consists of one marine genus, Seison, with two specie s (S. nebaliae and S. annulatus) which are epizoic on the crustacean N ebalia. Seisonidea are the only rotifers whose reproduction is strictl y bisexual. They also possess the unusual feature of having well-devel oped males. In this paper we review the literature concerning biogeogr aphical distribution, relationships with their host, and morphology, a nd present new information obtained with SEM and optical microscopy of both living animals and serial sections. Some seisonid characters, su ch as the paired retrocerebral organ, nervous system, and trophi, sugg est that they hold a primitive position within phylum Rotifera. Howeve r, they do share features with other rotifers: paired gonads and an un usual locomotory behavior with Bdelloidea; lateral antennae, cellular stomach wall, arrangement of intracytoplasmic lamina, and other charac ters with Monogononta. Some features are peculiar to Seison: obligator y amphimixis, endolecythal eggs, encysted spermatozoa, unique mastax, and absence of copulatory organ in males and of vitellaria in females. Reduction of the corona and absence of resistant stages in Seisonidea may be related to their habitat and to their life style. We propose a closer relationship of Seisonidea to Monogononta than previously asse rted.