A large number of case are known where sex is modified due to hormones secreted from sex organs. Three classical examples of hormonal or environmental control of sex would be presented here.
Sex in Bonellia
In Bonellia viridis, a marine worm, all larvae are genetically and cytologically similar. In this worm, male individual lives in the uterus of the female (Fig. 17.22). If a particular larva settles near proboscis of an adult female, it becomes a male individual, and lives in the uterus of the female. On the other hand, if it has to develop free in water, it becomes a female. Similarly, if a partly developed male is detached from the proboscis it become an intersex. Obviously the proboscis secretes a substance suppressing femaleness.