86 M. L. Plate on Ectoparasitic Rotatoria XII. — On some Ectoparasitic Rotatoria of the Bay of Naples. By LuDWiG Plate *. [Plate VI.] There are referred to tlie Rotatoria certain forms which depart widely from the ordinary type of this class and which have this in common in their mode of life, that they live as ectoparasites upon the marine Phyllopodiform genus Nehalia. Hitherto two different genera of these animalcules have been known, one of which was discovered by Grube f and intro-duced into science under the name of Seison. Subsequently Claus I recognized two distinct species {S. Oruhii and 8. an-nulatus) in the animals described by the above-named natu-ralist, and subjected these to a very thorough investigation. The second genus which must be referred to the group of the Seisonidaj was first described by P. J. Van Beneden § aud C. E. Hesse under the name of Saccohdella. One of the species discovered by these naturalists, Saccohdella nehalke^ was erroneously placed by them with the Hirudinese, and Claus first referred it to its proper place in the neighbourhood of the genus Seison. As, however, the two representatives of the latter differ considerably in the form of the body from Saccohdella nebalioi^ their generic distinctness must be main-tained, and it seems to me incorrect to cite the animal described by the Belgian naturalists as a distinct species of the genus Seison. How far the internal organization of 6'. nehalice agrees with that of the true Seisonidse cannot at present be decided, as our knowledge of that Eotatorian is still imperfect and in many points pressingly requires con-firmation. The genus Seison also needs further investiga-tion, because with regard to the sexual and excretory apparatus many questions still wait for a definitive solution. This circumstance induced me to devote a short sojourn at the Zoological Station at Naples in the spring of 1886 to the study of this interesting group of animals. * Translated from the ' Mittbeilimgen aus der zoologischeu Station zii Neapel,' Bd. yii. pp. 234-2U3. t E. Grube, ' Eiu Ausfiug nacli Triest und dem Quarnero,' Berlin, 1861, pp. 19 and 109-115, X C. Claus, " Ueber die Organisation und die systematische Stellung der Gattung Seison, Gr.,'' in the Festschrift zur Feier des 2.5-jahrigi'n Bestehen der k. k. zool.-bot. Ges. in Wien, 187G; and "Zur Kennlniss der Organisation \o\-\ tSeisou," in ' Zool. Anzeiger,' 1880, no. 68. § Van Beneden and Hetse, ' Recherches sur les Bdellodes ou I J irudinfes, et les Trematodes niarius,' Brussels, 1803, pp. 48 d seqq.
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