Invertebrates form an overwhelming majority of life on Earth, including crustaceans, insects, worms, corals, and more. The one fundamental similarity that all invertebrates share is the lack of a vertebral column. This is what separates them from mammals, birds, reptiles, and many fish. However, beyond this, the similarities quickly break down because the absence of a vertebral column is not taxonomically useful among such a wide and disparate spectrum of species. There are numerous huge phyla among the invertebrate population of the world, including mollusks, arthropods, and annelids.
There are several important marine arthropods, including all crustaceans: shrimp, crabs, barnacles and lobsters, but also other lesser-known subphylums such as pycnogonida, comprised of common sea spiders. The ancient and now-extinct trilobites were also arthropods, and the equally ancient but still living horseshoe crab is another marine arthropod worth mentioning because of its importance as an invertebrate with a nearly unchanged physical structure for the past 450 million years. Arthropods are known has being segmented into portions with exoskeletons growing over the outside of their bodies. These exoskeletons restrict growth, and, thus, all arthropods have to molt in order to grow beyond their confines.
The phylum of mollusks forms almost a quarter of all known marine organisms. They are highly diverse creatures divided into 10 taxonomic classes, of which squids and octopi are the most intelligent as well as the largest. The gastropods, which are comprised by snails and slugs, are the most numerous in terms of described species, and account for an overwhelming majority of extant mollusks. The most defining features of mollusks are as follows: the presence of a mantle with intake and outtake cavity, a radula for feeding, and a uniquely structured nervous system.
Annelids are very successful and important marine invertebrates; their phylum consists of worms and leeches of an astoundingly wide variety. Different species of annelids exist in nearly all waters and moist terrestrial ecosystems. Annelids can reproduce asexually and even regenerate after serious injuries. It is common for annelids to be soft-skinned and segmented into equal portions that each carry the same set of critical organs. This scheme of sharing segmented organs is what makes annelids so resistant to damage, and partially why they form such a widespread and successful phylum of invertebrate creatures. Annelids do not have eyes, but often feature cells that can detect changes in ambient light surrounding the creature.