The phylum Mollusca consists of over 112,000 species (2016) ¹ and is divided into eight classes, which are listed in the section headers below.These animals date back to the Paleozoic era.

476 families, 65,000 species (2017)

Includes: snails, slugs, limpets

Caenogastopoda, snails

Heterobranchia

Neomphaliones, limpets

Patellogastropoda, true limpets

Vetigastropoda, sea snails

106 families, 1,260 genera, 9,200 species

Includes: bivalve shell animals: clams, cockles, mussels, oysters, scallops

Heterodonta, 64 families, 800 genera, 5,600 species

Palaeoheterodonta, 7 families, 171 genera, 908 species

Protobranchia, 10 families, 49 genera, 700 species

Pteriomorphia, 25 families, 240 genera, 2000 species

940 species

Includes: chitons: marine animals with eight overlapping shell plates

Chelodida

Septemchitonida

Lepidopleurida

Chitonida

900 species

Includes: tusk shells, bottom dwellers

Dentaliida

Gadilida

EX Extinct

EW Extinct in the Wild

CR Critically Endangered

EN Endangered

VU Vulnerable

NT Near Threatened

CD Conservation Dependent

LC Least Concern

NL Not Listed

Before you raise an eyebrow at the use of "octopuses," you should know that octopuses and octopi are both acceptable uses as the plural of octopus. That′s another cool octopus fact!

Also, did you know that octopuses have tragically short life spans? Despite having three hearts, blue blood, and brains so developed they can use tools and solve complex puzzles, their lifespan is surprisingly short because as a rule, they mate once and then die.

After mating just once, females stop eating to guard their eggs and die shortly after the eggs hatch. Males fare no better and typically die within months of mating. Sadly, their short lifespan makes it hard for scientists to learn from these amazing creatures.

Blue-ringed octopuses, comprising the genus Hapalochlaena, are four extremely venomous species of octopus that can be identified by their yellowish skin and characteristic blue and black rings that change color dramatically when the animal is threatened.

The greater blue-ringed octopus is a species known as one of the most toxic marine animals in the world and is a small octopus whose size, including the arms does not exceed five inches and weighs about 3 ounces (80 grams). Its common name comes, not from the size of its body, but from the relatively large size of its blue rings, (about 3/8^th of an inch or 7 to 8 millimeters in diameter), which are larger than those of other members of the genus and which helps to distinguish this species of octopus.

A black line of variable thickness provides contrast to the electric blue rings, which number about sixty, are an aposematic adornment (an advertisement of the animals toxicity) to all potential predators. Also, by using a muscle contracting mechanism, this octopus will flash its blue iridescent rings (lasting about three seconds) so as to increase the warning to predators.

The head from front to back is slightly flattened and finished in a tip. Its eight arms are relatively short.

They eat small crustaceans, including crabs, hermit crabs, shrimp, and other small sea animals.

The animal prefers shallow waters with a mixed seabed (such as rubble, reefs and sandy areas). As is true for all octopuses, it lives in a burrow and only comes out to search for food or a mate. The entrance of the shelter is littered with remains from meals (empty shells and crab shell and legs) and is easily identifiable.

The greater blue-ringed octopus are seen in tropical and subtropical waters of the Indo-West Pacific region (from Madagascar to Sri Lanka to Philippines to Northern Australia) and found in coral reefs or tide pools within the Pacific and Indian oceans.

The greater blue-ringed octopus is capable of inflicting a deadly bite to its predators that can potentially be fatal to humans. The venom is called tetrodotoxin, a neurotoxin, which is found in several sea creatures including pufferfish.

Death in humans may occur within 20 minutes to 24 hours after onset of symptoms and usually results from respiratory paralysis.