CHORDATE ZOOLOGY STUDY GUIDE - M.O. Zakharenko - 2015

CHAPTER 3. DIVISION GNATHOSTOMATA

The division Gnathostomata, or Ectobranchiata, comprises all vertebrates except cyclostomes. In most Gnathostomes, the notochord exists only during the embryonic and larval stages. The Skull—either cartilaginous or bony—completely surrounds the Brain. Jaws are formed through the modification of visceral arches. Key characteristics of gnathostomes include:

- a jaw apparatus that played a major role in animal evolution;

- paired limbs, varying in Structure;

- a neurocranium that fully protects the brain;

- articulated gill arches;

- a slit-like Mouth opening that opens externally;

- paired olfactory vesicles opening via independent nostrils;

- three semicircular canals always present in the Inner ear capsule;

- aquatic gnathostomes possess unpaired fins In addition to paired ones;

- no more than five gill slits;

- ectodermal gills;

- the notochord Functions as a Skeleton only during the larval and embryonic stages. The division Gnathostomata is divided into:

Superclass Pisces (Fishes)

Cartilaginous fishes (Chondrichthyes)

Bony fishes (Osteichthyes)

Superclass Tetrapods, or terrestrial vertebrates - Tetrapoda (Quadrupeda) Amphibians (Amphibia)

Reptiles (Reptilia)

Birds (Aves)

Mammals (Mammalia).



Last update: 19/08/2026

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