У всех так. Но не без исключения. Среди плоских червей бывает очень много исключений из всего. Есть червь планария Dendrocoelum, такой свободноживущий плоский червь, гермафродит. Из Turbellaria. И есть рыбы Cynolebias. Из карпозубых Cyprinodontidae. Пресноводные такие рыбы. У них из зиготы образуются два разных эмбриона. А потом в ряде последовательных стадий они сливаются в одну гаструлу.
Dendrocoelum lacteum
Cynolebias
CA Carter, JP Wourms 1993 Naturally occurring diblastodermic eggs in the annual fish Cynolebias: implications for developmental regulation and determination // Journal of Morphology,
Annual fish development differs from that of other teleosts because a phase of blastomere dispersion-reaggregation spatially and temporally separates epiboly from embryogenesis. The fate of dispersed blastomeres was assessed in diblastodermic eggs of the annual fishes Cynolebias whitei and C. nigripinnis. In typical teleosts, blastomere determination and the events of primary embryonic induction occur prior to or during epiboly, so diblastodermic eggs produce partially or completely duplicated embryos. In the diblastodermic eggs of Cynolebias, the two blastoderms are completely separate from the one cell stage to the high blastula. Blastoderm fusion begins during midepiboly. By the end of epiboly, blastoderm fusion has been completed, and the deep, embryo-forming blastomeres of both blastoderms have completely dispersed and intermingled to form a single cell population. A typical annual fish dispersed blastomere phase ensues. Blastomeres reaggregate into a single mass, in which one embryo develops. When hatched, the young fish have no obvious structural or functional abnormalities. We suggest that the dispersed blastomeres of annual fish eggs are equivalent and that induction or determination takes place within the reaggregate. Alternatively, dispersed cells are partially determined but highly regulative, so that, when two populations fuse, the cells sort out according to tissue type and form a single embryo. In either instance, the formation of a single, normal embryo seems to corroborate the hypothesis that the dispersed cell phase of annual fishes is an adaptation that prevents environmentally induced developmental defects.
Это такое нормальное развитие. Сначала две бластулы, а потом сливаются.