• (n. pl.) A glass of fishes having a suckerlike mouth, without jaws, as the lamprey; the Marsipobranchii.
Cy·clos'to·mi noun plural [ New Latin See Cyclostomata .] (Zoology) A glass of fishes having a suckerlike mouth, without jaws, as the lamprey; the Marsipobranchii. 
The cyclostomi are a subclass of agnatha. They are modern eel-like creatures which eat fish.