Phylum Chordata

Four important characteristics found in all chordates at some point during their lifetime:

1) Notochord.

2) Dorsal hollow nerve cord.

3) Pharyngeal gill slits.

4) Post-anal tail.

Other characteristics typical of chordates:

        Bilateral symmetry


        Triploblastic with a developed Coelom

        Segmented muscle blocks

        Ventral heart


        Separate sexes

Thermal regulation strategies are highly diverse.

Chordates may be:

        Ectothermic (heat derived from outside the body),

        Poikilothermic (variable body temperature determined by the environment),

        Endothermic (heat derived from the animal's own oxidative metabolism),

        Homoeothermic (must maintain a constant body temperature).

Usually ectotherms are also poikilothermic and endotherms are homoeothermic, but some animals are hard to categorize using these terms.

Phylogenetic Tree of Phylum Chordata





Phylum Chodata

            Sub-phylum Vertebrata

Members of the Subphylum Vertebrata have the notochord replaced by a vertebral column composed of bone and/or cartilage.
The vertebral column, along with the cranium, limb girdles, and limbs, make up the endoskeleton. This internal skeleton is an adaptation for efficient locomotion, as was the notochord.

Class Agnatha: Jawless Fishes

Primitive fishes with a fibrous skeleton and an eel-like body.
Lack a jaw as well as the scales and paired fins we usually associate with fish.
Many species are parasitic (they attach to the outer surface of a fish with their sucker-like mouth).
Rasping teeth (arranged in a whorl) cut into the host. The lamprey then sucks blood from the wound (a fish hickey?). When it is finished its bloodmeal, the fish is released. The injured fish usually dies from bloo loss or infection.
Digestive system may lack a stomach
Key Characteristics of Agnathans
Median nostril at the anterior end (between the eyes)
Seven pairs of gill apertures 
While the lamprey may be either marine or freshwater, hagfish are exclusively marine. Unlike the lamprey, they are scavengers and are never parasitic. A hagfish's eyes are degenerate and eight tentacles surround its mouth. There may be five to 15 pairs of gill apertures, depending on the species.
Examples: Hagfish and Lamprey










Class Chondrichthyes: Cartilaginous Jawed Fishes

Examples: Sharks, Skates, and Rays

Key Characteristics:

Skeleton of cartilage
Paired appendages
Highly developed electro receptive system
Used to sense the electrical fields produced by all living animals
No swim bladder or lung Digestive system has a J-Shaped stomach with short intestine Separate sexes
May give birth to live young or lay eggs










Class Osteichthyes: Bony Jawed Fishes

Examples: Tuna, Salmon, Goldfish etc.

Key Characteristics:

Bony Skeleton (mineralized and ossified cartilage)
Mouth with teeth/jaw
Respiration supported by an operculum
A gill cap that moves fresh water over the gills without the fish having to swim
Swim bladder is present for buoyancy 2 chambered heart 1 atrium, 1 ventricle External fertilization with young hatching from eggs













 Transition to Land:

Fins need to be modified to walking limbs
Fins with cartilage for support become ossified
New method of breathing needed to be developed
Swim bladder modified to a primitive lung
Body needs to retain moisture
Adaptation not yet developed
This is why amphibians still remain by the water

 Class Amphibia

 Examples:         Frogs, Toads, Newts, Salamanders, Caecilians (limbless salamanders)

Key Characteristics:

Body Forms EXTREMELY varied
Usually four limbs, but some legless (Caecilians)
Smooth, moist skin with many glands (some poisonous)
No scales on skin
Respiration with the aid of true lungs
Aided with diffusion of O2 through the skin
Some forms keep external gills in adult form
Ectotherms Circulation by a THREE chambered heart
2 Atria
1 ventricle where oxygenated AND deoxygenated blood mix
Can do this because of skin diffusion + lungs
Separate sexes
Internal fertilization in Salamanders
External Fertilization in frogs

Class Reptilia

Further adaptations needed for land life

An egg with a shell for protection and prevention of water loss
Skin that is watertight
Skeleton that supports the weight of the entire body
Body mass is no longer supported by the buoyancy of the water
Skull modifications that allow better movement of the head and better vascularization (more blood flow)

General Characteristics

Dominated the Earth for 120 million years a far longer period of success than the more recent mammals.
Roughly 6000 living species
Outnumbers the combined number of amphibian and mammalian species
Exceeded by the number of bird and bony fish species
Body shape varied
Covered with epidermal scales
Paired limbs, usually with 5 toes
Absent in snakes and some legless lizards
No, a snake is not a legless lizard
Skeleton is well ossified with a sternum (breastbone) Respiration by lungs only, no functioning gills ever Some have a three chambered heart
Crocodiles and that line have a 4 chambered heart
Like Aves and Mammalia
Paired kidneys for nitrogenous waste removal Ectothermic with behavioral thermal regulation
This idea may be changing
Eggs covered with shells for protection
No larval stages present



No matter what Hollywood says, NEVER alive when man was alive
Not all dinosaurs were huge
One group, theoretically gave rise to the birds
Another, theoretically, gave rise to the mammals

Archaeopteryx possible link from reptiles to birds?









(Pic borrowed from

Therapsid possible link from reptiles to Mammals?











 (Pic borrowed from

Class Aves


Birds have been recorded closer to the North and South Poles than any other large animal
Can exist at higher altitudes than any other vertebrate animals (aside from humans)
Can dive deep into water (300 m) and come back up quickly without getting the bends (Emperor Penguins)

Keys to flight

General rules for wings
  1. Must move up and down
  2. Must move back and forth
  3. Must fold against the body when not in use
  1. Reduction and fusion of hand bones
  2. Hollowing of bones to lose mass but not strength
  3. Lighten the body
Aves adaptations
No teeth, nor muscles in the head to support and move a jaw with teeth
Skull bones fused together to make continuous sheet
Elimination of double organs

Characteristics of Aves

Epidermal covering of feathers, scales on legs
Paired limbs with fore limbs adapted for flight
Skeleton ossified but with air cavities
4 Chambered heart
Separate sexes with internal fertilization, but external development in a shelled amniotic egg. This aids in lightening the body.

Class Mammalia

Key Features

Bodies covered with hair
Used to retain heat
Mammary glands
Secrete milk for young
Even the most primitive mammals (Monotremes) have them
Give birth to live young
Only exceptions are the Monotremes
Start life with milk teeth, which are replaced by a set of permanent teeth Moveable eyelids with external fleshy ears 4 chambered heart Endothermic and homoeothermic Separate sexes with internal fertilization and development

3 groups (Infraclasses)

Monotremata (Prototheria)
Egg laying mammals
Duckbilled Platypus
Marsupials (Metatheria)
Birth to live young early and continued development in a pouch external to the womb
Placentals (Eutheria)
Full development in the womb
Insectivora Insect eaters
Chiroptera -- Bats
Lagomorpha -- Rabbits
Rodentia -- Rodents
Cetacea Whales and Dolphins
Carnivora -- Carnivores
Ungulata -- Horses
Primate Monkey & humans

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