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Phylum Arthropoda (joint-limbed)

The best known arthropods are the myriad terrestrial insects we encounter every day. Many marine arthropods can be found in our tidepools, though some of these arthropods are barely recognizable as belonging to the phylum.  These animals are generally grouped into the subphylum Crustacea, the crustaceans.  Like insects they have an external skeleton composed of chitin, though highly calcified.  They grow by molting (shedding the exoskeleton) and, in some cases, molting is also critical for reproduction.

Class Maxillopoda, Subclass Cirripedia (hair-foot): Barnacles

The "joint limb" is not at all apparent in the barnacles.  The "feet" are hairlike appendages that the barnacle extends through the slit at its top in order to trap small food particles.  The cirri are retracted in the two photos below.

Goose Barnacle (Pollicipes polymerus)

Acorn Barnacles (unknown species)

Class Malacostraca (soft shell)

Order Decapoda (ten feet): Crayfish, Lobsters, Crabs, Shrimp

Order Isopoda (equal feet): Isopods

Purple Shore Crab (Hemigrapsus nudus)	Lined shore crab (Pachygrapsus crassipes)
Rock crab (Cancer antennarius)	Ligia pallasii - a very large isopod (notice all of the legs)