It is thought that annelids were originally animals with two separate sexes, which released ova and sperm into the water via their nephridia. The fertilized eggs develop into trochophore larvae, which live as plankton. Later they sink to the sea-floor and metamorphose into miniature adults: the part of the trochophore between the apical tuft and the prototroch becomes the prostomium (head); a small area round the trochophore's anus becomes the pygidium (tail-piece); a narrow band immediately in front of that becomes the growth zone that produces new segments; and the rest of the trochophore becomes the peristomium (the segment that contains the mouth). COUNT 5
QSN1 How many sexes of annelids were there originally? ANS1 two QSN2 What releases sperm from annelids? ANS2 nephridia QSN3 What do annelid eggs become? ANS3 trochophore larvae QSN4 What do annelids' larvae live like? ANS4 plankton QSN5 What does the trochophore become when annelids mature? ANS5 the prostomium (head)