Vertebrates: Features and Characteristics

Characteristics of Vertebrates

• Vertebrates have a well-developed head due to cephalization
• They have a metameric body organization
• They have an endoskeleton
• They have a vertebral column (vertebrae) which supports the trunk
• The vertebral column is a series of metamerically arranged internal pieces of cartilaginous or bony skeleton
• The anterior part of the nerve cord is enlarged to form a brain
• The brain is protected by a brain case called cranium or skull
• The cranium has functions such as protection of the brain and orbits of the eyes, muscle attachment, and connecting the brain and other head organs to the rest of the body

Features of Vertebrates (contd.)

• Special sense organs called receptors are associated with the brain (e.g. eyes, ears, nose, tongue)
• The pharynx is small and the gill slits are few in number and often absent in adults
• Vertebrates have a chambered heart for rapid blood circulation
• They have a closed circulatory system with distinct blood vessels
• Red blood corpuscles contain haemoglobin to carry large amounts of oxygen
• Excretory organs are kidneys, which remove wastes and regulate the internal environment
• Endostyle is present in ammocoete larva of lampreys and is transformed into thyroid gland in other vertebrates
• Sexes are mostly separate, fertilization may be external or internal, and development may be direct or indirect through a larval stage

Classification of Vertebrates

• There are two superclasses of vertebrates: Agnatha (jawless fishes) and Gnathostomata (jawed vertebrates)
• There are six classes of jawed vertebrates: Chondrichthyes (cartilaginous fishes), Osteichthyes (bony fishes), Amphibia (amphibians), Reptilia (reptiles), Aves (birds), and Mammalia (mammals)

Superclass: Agnatha (Jawless Fishes)

• Common name: Jawless fishes or jawless vertebrates
• Examples: Lamprey and Hagfish
• Features: most primitive vertebrates, many agnathans are extinct, no jaws, true teeth are absent
• Body is elongated and eel-like in shape
• Living agnathans do not have scales; skin is slimy
• Adults are parasitic and marine, while larvae are free-living filter feeders on plankton, algae, and detritus in freshwater
• Hagfishes are exclusively marine

Features of Agnathans (contd.)

• Mouth of adult lamprey is circular and modified into a sucker with horny teeth and hooks
• Hagfish has tentacles round the mouth to detect food and a tongue-like structure with hooks to capture prey
• No paired appendages, but have fins which are not in pairs
• Single median nostril leads to a single median olfactory organ
• Skeleton is cartilaginous
• Embryonic notochord persists in the adult
• Gills are not protected or supported by gill arch
• Seven or more paired gill pouches are present
• Heart has two chambers
• Kidneys are adapted for excretion and regulating salt balance in both fresh and marine waters
• Most agnathans are hermaphrodites, fertilization is external