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the free-swimming larva of some annelid worms (polychaetes), echiuroids, sipunculids, and some mol-lusks. The microscopic body is fringed by one or more bands of cilia that facilitate locomotion in water. A preoral band or proto-troch is highly characteristic of the organism. At the upper pole of the trochophore is the sense organ, a parietal plate with a tuft of cilia and a group of nerve cells. The mouth is located on the ventral side, behind the prototroch. The intestine consists of a gullet, middle gut, and posterior gut. The anal opening is at the posterior pole. The excretory organs are a pair of protone-phridia. Alongside the intestine is a pair of primary mesodermal cells (mesoblasts), which produce a pair of mesodermal bands by repeated division. As a result of further development the trochophore acquires bristles, and its mesodermal bands become segmented, forming a series of paired coelomic sacs. The larva is then called a metatrochophore or nectochaeta.
A. V. IVANOV