Ampelisca abdita vs. Anpleisca vadorum

Ampelisca abdita (left) vs. Ampelisca vadorum (right)

What the top image leaves out, unfortuneately, is that A. vadorum's antennae aren't just longer, but they are much, much less setose. Also, as the bottom image indicates, the urosome has a distinctly different shape. "urosome segment 2+3, posterodorsal margin slightly concave; upturned distally." (LeCroy, 2002)

Sternaspsis!!! (photo by Michelle)

What you are looking at is the chitinized sheild on the posterior end of a polychaete from the family Sternaspidae. This family only contains one genus, Sternaspis.