This phylum represents the largest group of animals. More than 80% of the animal species that exist on the Earth are arthropods. This group includes insects, arachnids, and crustaceans. Specifically for the study of marine science the focus will be on crustaceans. Some of the common crustaceans include crabs, shrimp, barnacles, and lobsters.
All arthropods have hard exoskeletons, jointed appendages, and antennae which they use for protection, feeding, sensory reception and movement. Most of the crustaceans are decapod crustaceans which mean they have 10 legs. Crabs, lobsters and shrimp all have 10 legs. However, horseshoe crabs are not true crabs because they have 12 legs. In fact, they are more closely related to scorpions and spiders than to crabs.
Animals that possess a dorsal nerve cord, a rodlike notochord, and gill slits at some point in their development are called chordates and belong to the phylum Chordata. The lancelet and tunicate have these structures during development (only the lancelet retains all three in its adult form), but lack certain advanced traits; so they are classified in the subphylum Protochordates. The vertebrate chordates, organisms which have a backbone, include all fish, amphibians, reptiles, birds, and mammals. Therefore, protochordates represent an evolutionary link between vertebrates and invertebrates.
Lancelet (Cephalochrodata): tiny and transparent; fishlike in appearance; adult form retains dorsal nerve cord, notochord, and gill slits; lives half-buried in the sand, with head sticking out to filter plankton; has sexual reproduction (via separate sexes).
Sea Squirt (Urochordata): found worldwide; encrusts substrates such as the undersides of floating docks; squirts water when disturbed (hence its name); also called tunicate because of its clear, tough, outer membrane, or tunic; larval form has dorsal nerve cord, notochord, and gill slits, which disappear in the adult form; has sexual reproduction (hermaphrodite).