In the life history of Nereis trochophore larva is seen. It is an unsegmented, small ciliated larva. It has a broad anterior end and a narrow posterior end. The digestive system is curved- It opens through mouth on lateral side. It leads into oesophagus stomach, intestine. Above the mouth a pre oral band of cilia will be present. Below the mouth a post oral band of cilia will be present.
With the help of the cilia the larva swims. At its apex a apical tuft of cilia will be present. Below it a cerebral ganglion & 2 eyes are present. This larva will develop into adult animals after some time.