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Despite their names, fish are aquatic animals, lacking limbs that have digits. They include lampreys, hagfish, and cartilaginous and bony fish. There are several extinct species of related groups that are classified as fish. Here are some of these creatures: aquaria, carp, and mollusks. Listed below are some of the more interesting varieties of fish. Hopefully, this information will help you to decide which fish you want to keep as pets.

Historically, the term ‘fish’ was used to describe any aquatic vertebrate, although some ‘fish’ actually belonged to reptiles or mammals. These creatures have gills and fins and breathe through water. While all of them share the same ancestry, fish are a paraphyletic group in modern systematic biology. As a result, the term “fish” should be relegated to the animal kingdom and not be used to refer to any biological group.

Despite these similarities, fish are very different from tetrapods. In fact, fish are largely monophyletic, and their evolution stretches back over 450 million years. The word “fish” has no ancestry, meaning that it is a paraphyletic group that includes a wide variety of taxa. Among them are hagfishes, ray-finned fish, and sharks. The closest living relatives of tetrapods are the lungfish and coelacanths.