Dr. Frank Licher has posted all the information you'll ever want (minus figures which you can get from his book) about the polychaete genus Typosyllis, which he revised. Syllids are one of my favorite polychaete familes because they have all these crazy dorsal cirri on each parapodia (i.e. each segment of the worm). The dorsal cirri are the squiggly appendages on the worm to the left. Dr. Licher also posts a key to the genus, species descriptions (in german), a figure demonstrating the variety of the main identification character - setae. It is this last character that prompted one of polychaetology's most accomplished living expert, Kristian Fauchald, to note in his authoritative key to polychaete genera*, "... close examination of the structure of the setae is also necessary, making the identification of the syllids a time-consuming occupation." Dr. Licher also has a beautiful SEM (scanning electron micrograph) of a Typosyllis on his webpage that worth a look (click on the picture to see it enlarged and in full).
* Fauchald K. 1977. The polychaete worms: definitions and keys to the orders, families and genera. Natural History Museum of Los Angeles County, Science Series 28:1-188.
**Hat tip to Annelida listserve.