Vertebrate chordae have a number of advances over protocordates: brain mass protected by a cranial box and a segmented vertebral column.
The Vertebrata subphylum has approximately 40,000 living species and is the largest subphylum of the Chordata.
The approach we will take will preferably be related to the adaptations that exist in them and that favor their survival in the various environments in which they are found.
Agates or Cyclostomes: Primitive "Jawless" Fish
These animals have no jaw and have a circular mouth provided with a suction cup with horny teeth, with which they pierce the skin of the fish they feed on. The body of these fish is long and cylindrical, with the caudal part flattened laterally, and lined with thin, scaleless skin.
Lamprey and its Horny Teeth
The skin is rich in mucus-producing glands, especially in myxins, which produce it in large quantities to fend off predators. The skeleton is cartilage, as are the rays that support the paddle-shaped dorsal and caudal fins. They do not have even fins.
In lampreyes the notochord persists in the adult, surrounded by imperfect neural arches, being the support axis of the body.
In myxins this cartilaginous axis is even more incomplete (no more than a cord formed by cartilaginous nodules), allowing them to wrap their bodies in a knot, both to free themselves from predators and to feed.
When feeding, the myxins knot near the tail and move it to the head, forcing the mouth to pull a piece of meat from its prey.
Myxin or witchfish
The breath is made by gills, generally 6 to 16 pairs, in lateral gill sacs that open directly outwards into gill slits located near the head. Body temperature is variable - ectotherms.
O digestive system It has no stomach. The mouth is closed or opened by the backward and forward movement of the tongue, which also has the small horny teeth of the suction cup and is used to injure the prey, especially in parasitic individuals.
O nervous system It has a different brain, but the sense organs vary with the type of animal. Lampreyes have good eyesight but myxins are blind, although both orders have a refined smell and taste. The excretion is made by mesonephric kidneys.
Almost all agnátos spend their adult life at sea, migrating to reproduce, whether only for colder waters or even freshwater. In lamprey the sexes are separated and fertilization is external. The couples dig small shallow graves, where they lay the fertilized eggs and then die. The larvae - stools - they are very different from the adult form (they look like amphioxes), are blind and spend some time in the rivers (3 to 7 years), buried in sandy and calm areas where they filter their food because they have no teeth. They then undergo rapid metamorphosis and, as marine species, migrate to the ocean. In the case of myxins, these are hermaphrodites and the eggs have direct development, leaving the young of the eggs as miniatures of adults. Their reproduction always takes place in fresh water, where adults will also live.