All members of the Phylum Arthropoda have a distinct, rigid exoskeleton of chitin. Arthropoda are also known as the most taxonomically diverse phylum on the planet. They occupy nearly every known habitat on earth. Estimates state that there are anywhere from 30-100 million different species in this phylum. The segmentation of Arthropod bodies is different from that of Annelids in that they consist of subsections composed of fused segments referred to as tagmatization. Tagmata are the specific sections (i.e. Head, Thorax, Abdomen), which may vary from species to species. Some important features of arthropods are their open circulatory system, molting, coelomate, protostome and the fact that they live in almost all habitats on earth. Arthropods will molt their exoskeletons, this process of molting is also called ecdysis. Their exoskeletons have three main functions: protection from predators, prevention from desiccation, and locomotion (attachment sites for mussels). Sclerotization is the hardening of the procuticle after molting (also called tanning). Arthropods are also commonly noted for their jointed appendages. Examples of marine Arthropods include lobster, banded coral shrimp, cleaner shrimp. barnacles, horseshoe crabs, copepods, and other microcrustaceans that form the zooplankton.“Coral Banded Shrimp” by hjk_888 [CC BY-NC-ND 2.0]
The most important difference that is noticed between certain Arthropods such as crayfish and crabs is the morphology of and arrangements of body parts. Crabs and crayfish both have chelipeds, but crabs have a distinguishable single structure prosoma. As with all Arthropods, both types of creatures are contained by chitinous exoskeletons that they are able to shed and regrow throughout their life-cycles.“Rode amerikaanse rivierkreeft, Red Swamp Crayfish, Procambarus clarkii 04” by Luc hoogenstein [CC by 4.0]
Barnacles are interesting, even among the variant Phylum of life that is Arthropods. As with all other Arthropods, they are distinguished by a chitinous exoskeleton, however unlike many of their relatives, a barnacle also has an outer cup made of calcium carbonate that resembles a tiny volcano in structure. Barnacles also have internal calcareous plates that can close. This is not only for protection from predators, but also to keep the barnacles from drying out if they are not submerged in the water. There are over 1,400 known species of barnacles, most of which have the notable characteristics of secreting a natural ‘glue’ that allows them to adhere to the various surfaces that they inhabit. The incredible part is, through scientific observation and testing, the tensile strength of these natural epoxies have been measured at over 5,000 lbs/in2. The manner in which barnacles feed is through cirri. This is a ‘feather-like’ appendage that combs through the water for zooplankton and other microorganisms that they can capture and ingest.