1. Cercaria convoluta, a new species belonging to the Diplodiscinæ, is described. 2. The development of flame cells in both redia and cercaria of this species is based on a three-fold division of a primitively single flame cell on each side of the body. 3. A study of the development of flame cells in C. diplodisci subclavati, based on Looss's study, shows a four-fold group arrangement but with less exact symmetry. 4. A comparison of these two forms shows a fundamental difference of arrangement of a relatively conservative system within the same subfamily.