Gyrodactylus

These flukes are small, only a few mm in length and live on the body surface of fish. The most noticeable feature is the posterior opisthaptor, visible at the bottom of the photo. The opisthaptor has a pair of large anchors, or hamuli and some smaller marginal hooklets which are visible around the periphery of the opisthaptor. These allow the fluke to attach securely to the epithelium covering the scales. Since the opisthaptor is a non-replicated structure, it easily places this fluke in the Monopisthocotylea.

There are two lateral cephalic lobes at the anterior end of the worm, each containing cephalic glands that are not visible in the photo. These form a prohaptor. By alternately attaching with opisthaptor and then prohaptor, the parasite can move from one filament to another. The prohaptor in this species is not associated with the mouth. Posterior to the prohaptor is a dark-staining muscular pharynx which surrounds the mouth.

The only other prominent feature in the photo is a large, oval uterus which nearly fills the central one-third of the body. The fertilized embryo which develops within, will develop into a miniature fluke, which contains another smaller fluke developing in its embryonic uterus. This process of serial polyembryony allows rapid build up of the parasite population.