In certain cases environmental factors rather than chromosomes are known to influence sex as is seen in marine worm Bonellia, molluscs (Crepidula), daphnids,rotifers, aphids etc. The mechanism of sex determination in Bonellia has been studied by Baltzer.
The male and female individuals are strikingly dimorphic. The female worm is leaf like and several centimeters long. There is a long probosis extending from the anterior part of the body. The ovaries are distended.
The male individual is an extremely minute creature which lives in the reproductive tract of the female. The only function of the male is to fertilize the eggs produced in the ovaries of the female.
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Baltzer discovered that, if a single worm is reared separately from others, it invariably develops into female, while the worms patched together and released into water containing mature females tend to develop into both male and female worms.
Some of the young ones attach themselves to the proboscis of the female and obtain nourishment. These become male worms and eventually migrate down to the oviducts.
It is assumed that some hormonal secretion at the probosis region in the female transforms the attached young ones into male. In the marine gastropod Crepidula plana, Gould (1912) and Coe (1943) have described environmental influence on sex determination.
Young ones reared in isolation tend to develop into females, while those reared in groups in close contact with adults develop into males. This is also traceable to some chemical influence from the adult organisms.
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Whitney and Shull have reported that in rotifers, by changing oxygen level and nutrition supply, sex changes may be induced. In daphnids, crowded development tends to influence the formation of male individuals as reported by Banta.