The Chordates Also Known As… Animals With Some Real Spine!

What is a “Chordate”? A chordate is an animal with a nerve cord that runs along their back. All chordates have gill slits near their throats either in their adult lives or sometime in their developmental stages. All of them have a tail at some point in their lives or development. All chordates show bilateral symmetry.

Chordates & Vertebrates All vertebrates are chordates but not all chordates are vertebrates. There are two early classes of chordates called Urochordata and Cephalochordata. The Uruchrodata are small marine animals known as tunicates. The Cephalochordata are the lancelets (also small marine animals). Neither the tunicates or the lancelets have a backbone (vertebral column). They do have the nerve cord but no bony vertebrae to protect it so they are not classed as vertebrates.

What is a “Vertebrate”? A vertebrate is a animal with a backbone made of cartilage or bone which serves to protect the dorsal nerve cord. You will also see in the vertebrates: –A skull to house their enlarged brains. –Well-developed internal cavities that house the internal organs. –A ventral heart that pumps blood throughout a circulatory system. –Gas exchange through the use of gills or lungs depending on the habitat/lifestyle of the organism. Vertebrates include the jawless fishes, cartilaginous fishes, bony fishes, amphibians, reptiles, birds and mammals (seven classes).

Class Agnatha The agnathans are the jawless fishes. Members of this group include the lampreys and hagfish. There are very few agnathan species alive today. The lampreys are ectoparasites while the hagfish is a scavenger. AND DON’T DO STUFF LIKE THIS!

Class Chondrichthyes The chondrichthyans are the cartilaginous fish. Their skeleton is internal as is ours but it is made of cartilage rather than bone. The groups includes sharks, rays and skates. They have two sets of paired fins – pectoral and pelvic fins. Breathing is done as water moves over the gills as they swim – stop swimming and you die. Sharks are ovoviviparous – females keep fertilized eggs internally and give birth to live young.

Chondrichthyes

Class Osteichthyes Osteichthyans are the bony fish. Their internal skeleton starts as cartilage but it is replaced with bone. Flat, smooth, light-weight scales cover the body. They use gills to breathe but they are covered by an operculum – a bony plate that pumps water over the gills so bony fish can still breathe even when they are not moving. Most bony fish have swim bladders that help suspend the fish in water. The lungfish uses the swim bladder as a lung to breathe. Most fish lay eggs and release sperm externally and then leave. The sperm swim to fertilize the eggs. Fish hatchlings never know their real parents!

Osteichthyes

Class Amphibia The amphibians are an important evolutionary species as they were the first animals to leave the water and live on the land. All amphibians still need the water (usually freshwater) in their early stages but develop lungs to breathe air out of the water as adults. The frog has a three-chambered heart. Air is acquired by diffusion through the lungs, skin or lining of the mouth depending on the species. Amphibians reproduce very similarly to fish – another example of poor parenting! Common amphibians include frogs, toads, salamanders and newts.

Amphibia

Class Reptilia The reptiles include alligators, crocs, turtles, lizards and snakes. Reptiles picked up where amphibians left off with their adaptations to a terrestrial lifestyle. –They do not need water to reproduce. –They have eggs that have a hard shell to minimize water loss. –Fertilization is internal. –Tough, scaly skin restricts water loss. –Kidneys can form highly concentrated urine so water can be returned to the body. The lungs of reptiles are considerably larger than the amphibians.

Reptilia

Class Aves The avians are the birds. They have developed from reptiles and share a few characteristics –Scales on legs. –Large-yolked eggs in a firm shell. –No metamorphosis in young. Birds differ from the previous groups discussed: –They are endotherms – maintain an internal body temperature. (Others so far have been ectotherms.) –Feathers – which evolved from scales – they help preserve heat and allow for flight. –Hollow bones keep them light for flight. –Air sacs branch off from the trachea keep the bird respired and buoyant during flight.

Aves

Class Mammalia The mammals have the key characteristic of mammary glands which are used to feed their young. Other key traits of mammals: –Endothermic –Covered in hair or fur –Four-chambered heart

Mammalia

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