The genital papilla of the female acquires a great development during the breeding season and becomes produced into a tube nearly as long as the fish itself; this acts as an ovipositor by means of which the comparatively few and large eggs (3 millimetres in diameter) are introduced through the gaping valves between the branchiae of pond mussels (Unio and Anodonta), where, after being inseminated, they undergo their development, the fry leaving their host about a month later.
Gills or branchiae may be developed by parts of an appendage becoming thin-walled and vascular and either expanded into a thin lamella or ramified.
Some of the special modifications of branchiae are referred to below.
When present, the branchiae are generally differentiations of parts of the appendages, most often the epipodites, as in the Phyllopoda.
The podobranchiae are clearly epipodites, or, more correctly, parts of the epipodites, and it is probable that the arthroand pleuro-branchiae are also epipodial in origin and have migrated from the proximal segment of the limbs on to the adjacent body-wall.