I was taught that they are cartilaginous fish (elasmobranchs), alongside jawless fish and bony fish (gills & (optional) fins 'n that); ... has that classification changed?
That usage, not as far as I know, but it was pointed out in class that " fish"
is a term of convenience. What other word is there that suits
common usage?
They sell clams, crabs, sea cucumbers and, yes, fish at the fish store.
Term of convenience.
Sharks, hagfish, lampreys are so distinct from eachother and from
bony fish as to make for great imprecision calling them fish.
A thin case can be made for calling all vertebrates "fish"
if greater imprecision is desired. It's silly but-
why stop at "fish" I don't know since greater imprecision
is possible by invoking an earlier common ancestor,
some hemichordate or blatherskite or something.
Professor xx was so adamant about precise meanings
when he taught the course, including "sharks are not fish",
I was his student, thought he made sense then and do
now, regardless of such word games as suit persons
of lesser note.