1. The structure, renewal, embryonic development, and regeneration of the swimming setae of Daphnia carinata are described.
2. Each seta is formed by four giant cells, two (the core cells) forming the distal segment, and two (the sheath cells) the proximal segment. The inner ends of these cells extend back, as the seta strand, through the whole length of the rami of the antenna to be inserted into the hypodermis half-way down the protopodite.
3. The proximal segment of the seta is formed as an inverted sac, enclosing the distal segment. At ecdysis, the sac is everted.
4. The mechanics of the extrusion of the new setae at ecdysis are described.
5. In embryonic development the seta-forming cells are greatly enlarged hypodermal cells, which grow back from the tip of the antenna to the protopodite.
6. The primary embryonic setae consist of the distal segment only of the adult seta.
7. If a ramus of the antenna is amputated, the missing segments are never regenerated. Formation of new setae takes place, however, from the hypodermal membrane which grows across to close the wound. These setae are formed in the same way as in embryonic development.
8. The muscles in the segment through which the amputation was performed degenerate and are not replaced.
9. The potentiality of enlarging and becoming a seta-forming cell is possessed throughout life by all the hypodermal cells of the two rami of the antenna, but not, apparently, by the cells of protopodite hypodermis.