PROCEEDINGS ON THE UNITED STATES NATIONAL MUSEUM hy the SMITHSONIAN INSTITUTION U. S. NATIONAL MUSEUM Vol. 99 Washington : 1949 No. 3245 A EEVIEW OF THE COPEPOD GENUS PARANTHESSIUS CLAUS By Paul. L. Illg The problem of arriving at a natural classification of the copepods has been in no small measure complicated by the repeated appearance within the group of the parasitic and commensal habit. In the cyclo-poids, two sections, the Poecilostoma and the Siphonostoma, have been established and remain well differentiated on the basis of the special-ization of mouth appendages correlated with parasitic and semipara-sitic existence. Unfortunately for the systematist, these copepods seem to exhibit the widest tolerance in the selection of hosts. From the records, any phylum above the Protozoa which is represented to any conspicuous degixe in the marine fauna is likely to appear in the roster of hosts of cyclopoid symbionts. This diversity of occurrence, in addition to small size, inconspicuous appearance, and remarkable agility in eluding usual methods of capture, has resulted in a scattered and fragmentary representation of this group in museum collections and consequently in the literature on copepods. The most extensive reports to date either have been surveys of the faunas of fairly limited localities or have been compendia of the symbionts of some particular category of host, with the emphasis of interest frequently tending toward the host rather than toward the copepod. Probably a high degree of artificiality has been introduced into the classification because of this history. One of the major groups of the copepods is now found defined in numerous publications as an assem-blage of parasites and commensals of ascidians. This reference is. of course, to the Notodelphyoida, which in such broad interpretation 818707 — 49 1 391
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