ulcer:

a localized breakdown of epithelial tissues.

unarmed:

lacking spines, hooks, or other hardened structures.

undulating membrane:

a region of some Mastigophora in which the surface pellicle is drawn up into a fin-like ridge by the flagellum.

unilocular hydatid:

a hydatid cyst with a thick, external cyst wall and asexually budding in internal brood capsules only.

unembryonated:

an egg prior to the differentiation of the zygote into an embryo.

unilateral:

an arrangement in which one type of structure (such as genital pores) occurs consistently on one side of the body.

unsporulated:

not yet having undergone sporulation.

uterine bell:

a funnel-shaped organ in female Acanthocephala that collects eggs floating within the body cavity and passes them posteriorly to a selector apparatus.

uterus:

duct in female reproductive system used to conduct eggs towards the genital opening.