1) Considerable numbers of pure-bred cultures of bothStrongylocentrotus Purpuratus andS. Franciscanus were raised during the experiments. In addition, eggs of both species were made to develop parthenogenetically, and the two kinds of hybrid-plutei were raised,S. Purpuratus ♂/S. Franciscanus ♀, andS. Franciscanus ♂/S. Purpuratus ♀. Eggs of both species were also fertilized with sperm of the starfishAsterias ochracea, and hybrid-plutei produced from this union,Asterias ♂/S. Purpuratus andAsterias ♂/S. Franciscanus ♀. 2) Pure breeds as well as all the hybrids showed a certain latitude of variability in the form of the skeleton of the pluteus. On comparing large number of plutei of the four different kinds raised from the eggs of each of theStrongylocentrotus species, this variability proved to be within the same limits in all the different breeds. 3) The different breeds of hybrids were without exception always purely maternal. No difference whatever could be detected between the plutei of the pure breeds ofS. Purpuratus, the parthenogenetic plutei ofS. Purpuratus, the hybrid pluteiAsterias ♂/S. Purpuratus ♀ and the hybridsS. Franciscanus ♂/S. Purpuratus /pF. The plutei of the pure culturesS. Franciscanus differed just as little from those of the hybrid culturesAsterias ♂/S. Franciscanus ♀, or from parthenogenetic plutei ofS. Franciscanus.