Louisella.

Louisella was a priapulid worm that has been found in the Burgess Shale in British Columbia. It was one foot long, which meant it was the largest priapulid in the Burgess Shale. 

Louisella was a slender worm with feathery papillae running down the bottom of its body in two rows. These were possibly gills. It had a long proboscis on its head. Before the proboscis it had a ring of spikes pointing forward, which I think kind of looks like a crown. 

Like most Cambrian priapulids, Louisella was a predator, consuming small creatures such as hyoliths and swallowing them whole, front first. If Louisella swallowed hyoliths from the rear end, the spines of the hyolithid would get stuck in the worm's throat, possibly stab it, and prevent Louisella from eating any more hyoliths. Unlike its ferocious relative Ottoia, Louisella presumably was not as active, but it was almost certainly more active that Selkirkia, which would have found it hard to move in the first place.

© MARIANNE COLLINS

I believe that Louisella's large head spines prevented other priapulids from swallowing it while Louisella was small. The only priapulid that would have been likely to try to eat it would be Ottoia, which swallowed its prey head first. And if for some reason an Ottoia tried to swallow Louisella from the rear end, Louisella could just turn around and stab it. These spines would probably work against any other predator that tried to eat it. Even an arthropod would let Louisella go if it was stabbed in the right place, such as a joint or, as in the case of some arthropods, its softer underbelly. This is similar to the defense of Fieldia, which had a whole head covered in long, needle-like spines. But this is just my hypothesis about the use of the large spines on Louisella's head.

© SMITHSONIAN INSTITUTION – NATIONAL MUSEUM OF NATURAL HISTORY. PHOTOS: JEAN-BERNARD CARON

Some of the fossils of Louisella are very slender, while others are flatter and more sea cucumber-like. The holotype was one of these flattened specimens, which is what probably led Charles Doolittle Walcott to classify it as a holothurian, or sea cucumber. 

FOSSIL MALL

References:

http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=75&ref=i&

http://www.paleo.pan.pl/people/Dzik/Publications/PaleobiologyDzik.pdf

http://www.fossilmall.com/EDCOPE_Enterprises/invertebrates/invert12/invfossil-12.htm

The Crucible of Creation: The Burgess Shale and the Rise of Animals by Simon Conway Morris

http://en.wikipedia.org/wiki/Louisella