|Acorn barnacles on a stone, feeding. The inside of the shell is a deep red.|
|Long, pale legs, and shorter dark green legs. Another barnacle in the background, smiling.|
A barnacle has 6 pairs of legs, putting them between the decapod crabs and shrimp, and the 14-legged isopods. They should really be called "dodecapods" (from the Greek δώδεκα dōdeka "twelve" + ποδός "foot"), although we usually forget that those waving fans are actually feet, modified into feathery cirri, to capture food and absorb oxygen from the water.
And then there is the thirteenth appendage:
|The same barnacles. One on the lower right is starting to extend his long penis. It's hairy, too, but not striped.|
Each barnacle is hermaphroditic, which is handy, because they can't wander about in search of a suitable mate. Any other barnacle will do, as long as it is within reach of that long, extensible penis.