Scyphidia tholiformis, a peritrichous protozoan found on the gills and external surfaces of Micropterus dolomieu and Micropterus salmoides

Scyphidia tholiformis is an urn‐shaped peritrichous protozoan measuring about 59 microns in length and 35 microns in width. It possesses a double row of cilia about one‐third of the distance from the anterior end of the body and has a large scopula which is variable in shape and provides a large surface for attachment. The macronucleus is long and band‐like, and makes a semicircle beneath the peristome from which it extends into the lower part of the body. The small, ovoid micronucleus is located near the middle of the body.

The organism is found on the external body surfaces and the gills of large‐mouth and smallmouth black bass. Its relationship to its host is probably that of an ecto‐commensal, but there is further evidence that these organisms may indirectly cause mortality among bass.

The organism is interesting because of the manner in which it becomes dome‐shaped when it leaves its host, causing it to resemble Trichodina. In this form, locomotion is accomplished by means of the central cilia.

It reproduces by longitudinal binary fission and cojugation of anisogametes.