Marine Invertebrates are organisms that lack a notochord, or backbone, and live in saltwater environments. Accounting for over 50,000 of Earths species, marine invertebrates can be found in almost every marine habitat on the face of the earth. They range in size from the Colossal Squid (Mesonychoteuthis hamiltoni) which is estimated to be over 45 feet long to micro-organisms measuring in at less than one millimeter. To make up for their lack of spinal structure they usually have a hard external shell or exoskeleton. Those that do not have an exoskeleton often rely on outside forces such as hydrostatic pressure for support. For defense, marine invertebrates without an exoskeleton, such as jellyfish, often utilize stinging cells and/or unpalatable taste to ward off predators. Marine Invertebrates including filter feeders, herbivores and detritivores are often the "clean-up crew" of the home aquarium.