ABSTRACT
In animals, echinoderms especially the echinoids have served as a model for the study of fertilization and early embryonic development. The interspersion of asexual reproduction within sexual reproduction in many holothuroids, asteroids and ophiuroids has attracted much attention. In gonochores, the benefits arising from recombination during gametogenesis and fusion of two gametes from two sexually different individuals during fertilization increase genetic diversity, the raw material for evolution. Within gonochoric echinoderms, sex ratio seems to be strictly maintained at and around 0.5 $: 0.5 J. Spatial and temporal changes in the ratio due to sex-dependent mortality and movement skew the ratio slightly. Reproductive barriers between species are classified into prezygotic and postzygotic categories. Among aquatic metazoan invertebrates, echinoderms display the simplest reproductive system. In some of the hermaphroditic echinoderms, sexuality ranges from selffertilizing simultaneous hermaphroditism to sequential hermaphroditism.
