|LImpet. The yellowish bits are limpet poop.|
Limpets are built like snails; because they have retreated into a shell that opens only on one side or one end (snails), their anatomy has to be twisted away from the "normal" head to tail shape of other animals. For example, they have two kidneys, as do we. But the left one is tiny, because it just wouldn't fit otherwise. On the exposed bottom of a limpet, we see the mouth, two tentacles, with their eyes, the big foot, with the gills laid alongside. (Not visible in this photo.) And the anus is up near the head. Which could be a problem.
The anus of most molluscs and indeed many animals is located far from the head. In limpets and most gastropods, however, the evolutionary torsion which took place and allowed the gastropods to have a shell into which they could completely withdraw has caused the anus to be located near the head. Used food would quickly foul the nuchal cavity unless it was firmly compacted prior to being excreted. (Wikipedia) (My emphasis. "Nuchal" means near the neck.)
|One of the tinier hermit crabs. The eelgrass is about 1/4 inch wide; half its width is visible here. So the hermit, shell and all, is about 3/8 of an inch long.|
|Channelled dogwinkle, Nucella canaliculata. It eats barnacles, prefers mussels. I don't bring many mussels home for them, though; they (the mussels) trap and kill my hermits.|
|Channelled dogwinkle finishing off a stonefull of barnacles. There is one still alive, still cheerfully trolling for supper, at the bottom right. Sometimes I feel sorry for them.|
|Bejewelled (or at least, be-sanded) Japanese nassa, Nasarius fraterculus. The fine sand must be stuck to the algae growing on the shell. Average sand grains beneath it: these are not stones.|
|Knobbly column of a pink-tipped green anemone. The neighbours are a limpet, a couple of Asian mud snails (invasive Batillaria), a hermit, and another pink and green anemone. There are 9 of these in the tank now; they keep cloning themselves.|
And the chiton is still on his moon snail shell.