Vertebrates are animals with backbones that are a sub group of animals in the Craniata phylum.
The subphylum Vertebrata is placed within the phylum Chordata. Most chordates are bilaterally symmetrical animals with differentiation into head, trunk, and tail.
As a rule, chordates are active animals. The most distinctive anatomical features of chordates are a notochord and nerve cord. The notochord is an exceptionally important characteristic of chordates. It is like a stiffened rod that does not compress. This allows the body muscles to act against the notochord and thus allows the animal to move.
The phylum Chordata includes three subphyla:
- Subphylum Acrania includes about 30-35 contemporary species placed in one class and three families. All are marine animals. Look like small (approximately 10 cm or smaller) semi-transparent fish.
- Subphylum Urochordata (Tunicata) consists of three classes of exclusively marine animals.
- Class Ascidia includes about 1,000 contemporary sessile filter-feeding animals also called Sea Squirts. Order Synacidiae includes species living in colonies, while order Monoscidiae includes solitary animals.
- Class Thaliacea (Salpae) includes about 70 contemporary pelagic filter-feeder species shaped like a barrel, and is divided into three orders. Order Pyrosomidae consists of species living in colonies: orders Salpidae and Doliolidae consist of solitary species.
- Class Appendiculariae includes about 60 small (several millimeters) pelagic species.
- Subphylum Vertebrata includes animals with a distinct internal skeleton. They are multicellular animals derived from embryos that have three cellular layers: endoderm (endo- internal), mesoderm (meso- medium) and ectoderm (ecto- external). They have bilateral symmetric bodies, and internal gut with two openings, mouth and anus.
Only vertebrates have a true brain with several different areas and a skeletal structure that protects the brain, the cranium. They have developed sensory organs (eyes, ears, olfactory organs).
They possess a more complex digestive system, with several accessory digestive glands. The heart is chambered. They have developed more complex respiratory and muscular systems as well.
Classes within Vertebrata include:
• Cyclostomata (Lampreys and Hagfish)
• Chondrichthyes (Sharks, Skates and Rays, Elephant Fishes)
• Osteichthyes (Bony Fishes) (Choanichthyes (Lungfish) separated from this class by some researchers)
• Amphibia (Amphibians – Frogs and Toads, Newts and Salamanders, Caecilians). Not associated with marine environments due to permeable skin. One exception is the crab-eating frog Fejervarya cancrivora living in mangrove estuaries of Southeast Asia.
• Reptilia (Sea Turtles, Crocodilians and Sea Snakes)
• Aves (Sea Birds)
• Mammalia (Sea mammals such as sea otters, whales, dolphins, seals and walruses).