What are fish? Fish is a general term for a taxonomic group of vertebrates excluding tetrapods
(four-legged animals）. Most
of the “fish” we imagine are teleosts but, with a few exceptions, this
definition is as ambiguous as the one above. Exceptions include lampreys,
sharks, rays, coelacanths, and lungfish. The sturgeon and bowfin, which are
called ancient fish,do not belong to the teleosts.Let me explain this in a little more detail.
What are vertebrates?
Animals that have a brain and centrally manage neural
information include mollusks, arthropods, and vertebrates. On the other hand,
animals in which neurotransmission pathways are scattered throughout the body
include cnidarians and echinoderms, such
as sea urchins and starfish[A1]. Mollusks and arthropods have transmission pathways on the
ventral side, whereas chordates have transmission pathways bundled on the
dorsal side. The notochord is connective tissue (supporting tissue, derived
from the mesoderm) that replaces the spine.
It has the function of inducing neural tubes (which later
develop into the spinal cord) during vertebrate development. It normally
degenerates later in the process of development (it is replaced by the spine). However,
the spine is underdeveloped in cyclostomata and sturgeons, and the notochord even in adults.
