Two new species of Gyrocotyle (Monogenea from Chimaerids (Holocephali))

INTRODUCTION Species of the genus Gyrocotyle are quite common intestinal parasites of chimaerids. As far as we now know, each parasite species is restricted to one host species. In each host species, however, a second and much rarer species of Gyrocotyle may occur besides the common one. The common species are systematically related to each other, and so are the rare ones. In the present paper two species of the group of rare species are described. Their existence has been known already for a long time, but, for different reasons, they have never been recognized as separate species. DEFINITION OF SOME TERMS Ι. Spines. — We recognize two types of spines in the genus Gyrocotyle, viz., the urna-type and the confusa-type. In spines of the urna-type the general shape is stubby, irregular and asymmetrical, and the distal portion is stout and blunt (fig. 5). In spines of the confusa-type, the general shape is elegant, regular and symmetrical, and the distal portion is slender, gradually tapering, and pointed (fig. 2-4). In spines of the urna-type the points that project beyond the skin are twice as wide as in spines of comparable dimensions of the confusa-type, which is even striking when specimens are studied under a low-power stereomicroscope. 2. Lateral ruffles. — All species of the subgenus Amphiptyches have lateral ruffles. A complicated type, with a great dorsoventral amplitude and with a few to many secondary undulations, can be distinguished from a simple type, generally with a small dorsoventral amplitude and without secondary undulations. 3. Rosette. — All species of the genus Gyrocotyle have a rosette, i.e. a