First a quick detail of English: A net full of tuna has "fish" in it. "Fish" is the plural, if you are speaking of many wet scaly things all of the same species. If you want to say "In this net there is tuna, mackerel and cod", you can say "This net holds fishes", to convey the fact that there are several species present. You may say "This net holds fish.", if you don't care about the types, you just want to say there is more than one wet scaly thing in the net.
If English is not your first language, you have my sympathy and admiration. How did such a terrible language become so dominant in the world?
When I told you that the fishes is one group (order) of the vertebrates (class), I lied.
The things that swim in the waters of the world, salt and fresh, with paired fins and scales are indeed fish... but they are not all in one order, and some would say that not all of them should be called "fish".
Whether you believe that the panoply of living things was created in a few days by God, or you believe the theory of evolution... and there are many, sincere people in both camps... you might want to consider the discoveries emerging as scientists look at the DNA of different organisms.
The DNA evidence suggests that we might want to consider grouping the fish-like things into three groups...
The list above leaves out quite a few groups... but will do for a start!
The word "fishes" is in quotes in the first two cases, as not everyone believes that the term should be applied beyond the bony fishes group.
You might be interested to learn that the DNA evidence suggests that the bony fishes are more closely related to elephants than to the lampreys... but don't worry over-much about that yet!
The bony fishes sub-group contains nearly 30,000 species. It contains most of what you would think of if I said "fish".... and probably plenty of things you would not think of. (You could spend a lifetime just studying the bony fishes.) The fact that these vertebrates have skeletons made of bone is one of the main things setting them apart from the other fish-like organisms. The fancy name for the group is the osteichthyes. That link will take you to Wikipedia's page. (Some of those "not obvious" fish: Sea horses. Eels.)
The cartilaginous "fishes" sub-group contains sharks, rays, skates. Their skeletons made of cartilage rather than bone. (If you encounter some gristle when eating some meat, that is an example of cartilage.) The fancy name for the group is the chondrichthyes. That link will take you to Wikipedia's page, from which the photo on the right comes... thank you user "Drow_male", who is a Spanish speaker, so perhaps "Drow_male" means more when translated.
The jawless "fishes" sub-group contains the lampreys. At first glance, a lamprey looks quite like an eel, but there are significant differences when you look more closely. A few species of lampreys... remember they are eel-like to look at... use their rather fearsome mouths to attach themselves to big fish, from which the lamprey then sucks blood. Yuck! But everyone has to make a living somehow, and at least these lampreys have avoided being "boring". You may think that you have no use for biology; you may only be here because someone has forced you along. But think what you could do with an idea like that if you wanted to be a maker of horror films? (Note I said "an idea like that". Lamprey- like sucker things have appeared in films already. Go find something else to put in your film. The fancy name for the group is the hyperoartia, or petromyzontida. That link will take you to Wikipedia's page.
Did you notice I've been eschewing the normal group terms "kingdom", "phylum", "class", "order"...?
While the usual taxonomic heirarchical naming system is used by icthyologists (fish scientists), they've "fine tuned" the system a bit, adding a few layers.
A mackeral is still in the animal kingdom, chortates phylum. It even has a family, genus and species in the usual way. It is "in between" that professionals today no longer exactly use the nice simple "vertebrates class, fish order" system. But you can! You don't need to sub-divide the sub-divisions. You are (I hope!) learning a scheme of living things that allows you to quickly run through "all the possiblities" in an organized manner. And for that, "vertebrates class, fish order" will do fine.
This page is still under construction. Complain if this note hasn't been updated by 22 October? I hope you will agree that what is above is worth something as it stands??
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