troch-, trocho- +

(Greek: that which is round[ed]; a wheel, a disk)

1. The small, free-swimming, ciliated aquatic larva of various invertebrates, including certain mollusks and annelids.
2. The free-swimming, pear-shaped, generally translucent larval form of marine annelids, mollusks, and related invertebrates.

Externally it has a prominent circlet of cilia, internally a complete digestive tract, a pair of excretory tubes, muscle and nerve fibers, sense organs, and a band of mesoderm.

A young larval form of many annelids, mollusks, and bryozoans, in which a circle of cilia is developed around the anterior end.