Sexual dimorphism is seen in many species, with distinct male
and female zooids interspersed with autozooids, and most brood
chambers, or ovicells, consist of single or multiple polymorphic
units. Avicularia and vibracula are highly modified polymorphs in
which the zooid operculum forms a mandible or seta; these serve
defensive functions, repelling micropredators and discouraging
settlement of invertebrate larvae, and dispersing silt and debris.
Class Phylactolaemata comprise most freshwater ectoprocts, such
as Plumatella (Figure E) and Pectinatella magnifica (Figure F) are
uncalcified. In many species the lophophore is U-shaped, but no
species displays polymorphism. Colony form is limited in this
class, from simple repeat, branching chains to spongy or gelatinous
mounds; most are sessile, but a Cristatella colony can creep
with its muscular shared foot. Phylactolaemates produce asexual
buds called statoblasts, that develop in summer on the funiculus
as balls of cells enclosed within paired chitinous valves, usually
disk shaped and often with a marginal float and hooked spines. In
the fall, statoblasts are released as the colony disintegrates and may
be dispersed by wind and water or attached to the breasts (perhaps
feet) of waterfowl. Statoblasts survive desiccation and winter
cold, and in spring, the valves open and a new zooid grows from
the enclosed cells to found a new colony. Statoblasts are unique to
phylactolaemates; formation of this resting overwintering stage is
ecologically comparable to tardigrade (A-27) resting eggs, rotifer
(A-14) resting eggs, and freshwater sponge gemmules (A-3).