These pictures show a 14-day-old metatrochophore larva of the polychaete worm Serpula columbiana, which we raised in the Embryology class. The larval anterior is rotated up. These larvae have several segments with chaete (chitinous bristles), a prototroch (pre-oral ciliated band) which is used in locomotion and feeding, and a metatroch (a post-oral ciliated band). The first photo is a dorsal view. You can see two large black eye-spots marking the anterior end of the larva. The gut is reddish-brown, and the circle near the posterior end of the larva is the anus. Several chaete are also in focus.
The second photo is a lateral view, ventral to the left. One can see the entire digestive system. The mouth opens ventrally between the prototroch and the metatroch, while the anus is dorsal. One can also see a small apical tuft at the anterior end of the larva. The large clear vesicle posterior to the anus is called the anal vesicle, but I do not know its function.
The third photo is a ventral view, which shows particularly well the long prototrochal cilia, the mouth, and the metatroch. The last photo is a close-up of the larva’s ciliated food groove located between the prototroch and the metatroch. The cilia of the prototroch and metatroch beat in opposite directions, and the larva feeds by trapping microscopic particles between these two ciliary bands, then moving them along the ciliated food groove to the mouth. This is referred to as the “opposed-band” feeding mechanism, which is characteristic of many trochophore-type larvae. See the trochophore stage of this species.