The protonephridia of the females of three species of Asplanchna are described and compared. Each protonephridium consists of a single unbranched coiled tubule, divisible into a distal capillary tubule receiving ducts from the flame bulbs and a wider proximal main tubule entering the contractile vesicle. Each flame bulb is of flattened triangular form with longitudinally-striated thin lateral walls and a thick terminal wall bearing internally the vibratile flame and externally a number of external filaments anchoring the flame bulb to the body wall.

The numbers of flame bulbs in the species of Asplanchna are compared. Information available suggests that there is a close correlation between the number of flame bulbs and the surface area of the animal. The surface area per bulb is less in males than in females of the same species and there is some evidence that the number of bulbs in males is less than that of the larger females.

Females of species of differing body size have flame bulbs of similar length and the estimated output of fluid per flame bulb is similar in each. It is suggested that an increase in number rather than size of bulbs accounts for the increased output from the contractile vesicle of the larger species, and that there is an optimum size for the flame bulb. The output per bulb in males is less than that of females of the same species and it is suggested that the absence of a gut in males may account for this difference.