Barnacle!

Barnacle

Pictures care of http://www.silvers.org/barnacle/barnacles.html

One might be forgiven for thinking of barnacles as mollusks rather than crustaceans. They are frequently mistaken for limpets and were in fact classified as mollusks right up until the nineteenth centur when an English naturalist identified their larvae; these larvae are free-swimming and do not attach themselves to focks or sips' hulls until they have almost reached the adult stage. When underwater, they eat microscopic animals like phytoplankton and zooplankton, so naturally, if they're above water, they have to wait until the tide comes in for their meal.
There are many different species of barnacle which fall into one of two categories, namely the acorn barnacles, which look like little limpets, and the goose barnacles, which have a longish, flexible 'foot' (just like what snails have to get around). This foot is really their head, barnacles being very odd in that their point of attachment is their head.
Barnacles attach themselves to any underwater object, including the carapaces (shells) of crabs and turtles and the flanks of certain whales. They are a recurrent nuisance to ships; colonies of them give unwanted ballast (something that helps a ship stay level to ports when loading and unloading, to balance ships... you'll learn about it in science class) to the hull and prevent the ship from gliding freely through the water, thus cutting down her speed. One problem is that their shells are very sharp, but the biggest problem is that they stick on solid items and stay there forever.
The sole occupation of these absolutely sedentary crustaceans is the non-stop filtering of sea water for the purpose of extracting edible particles. Most barnacles are hermaprodite (that is, each barnacles produces both male and female gametes), a condition associated with their sessile life, which prvents contact between individuals. Because they settle gregariously, cross-fertilization rather than self-fertilization is the rule. Some species of barnacle inhabit the inter-tidal zone where they are expised to the air for long periods.
There is one type of parasitic barnacle that attaches itself to the tidemarsh green crab. The baby swims around till it finds the green crab, then it attaches itself to the crab's shell. It then puts parts of its cells through the crab with its coating. Eventually these cells get into the bloodstream of the crab and come to rest in its stomach. The barnacle continues to grow inside the shell eating all the interior organs of the crab. By the time the crab sheds its old shell, a process called moulting, the parasitic barnacle is living inside of it.
The green crab isn't a green crab anymore. It doesn't lay crab eggs. It lays sacculina barnacle eggs. It looks like a green crab but it isn't. It has been taken over by the barnacle.

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