Chordates and Vertebrates

Introduction to ChordatesChordates are animals that have four structures present at some stage of their development.

cat embryo at 2 weeksnotochord

postanal tail

nerve cord

pharyngeal pouches or “gill slits”

Animals in this group include tunicates, lancets, fish, amphibians, reptiles, birds, and mammals.

All vertebrate embryos look very similar too each other because they are all chordates.

Note that all of these chordates have: a notochord, postanal tail, nerve cord, and pharyngeal pouches.

NotochordAll chordates have an internal notochord that supports the animal and extends along the upper part of the body (what we think of as the back.

- The notochord is a firm but flexible structure made up of fluid-filled cells that are enclosed in a stiff covering.

- The notochord extends into the postanal tail which is a muscular structure at the end of the developing chordate.

- In most chordates (fish, amphibians, reptiles, birds and mammals) the notochord gets replaced by the back bones or vertebrae

- Other chordates such as the tunicate and the lancet, the notochord is kept into adulthood.

Tunicates a.k.a. sea squirts are sessile marine chordates.

As larvae they are free swimming and contain notochord, nerve cord, and other chordate features.

Tunicates and Lancets

Lancets a.k.a. amphioxus are primitive eel-like chordates that spend most of their time buried in sand.

They contain typical chordate features.

Both tunicates and lancets

are invertebrates.

Nerve CordAbove the notochord is a tube-like structure called the nerve cord.

- As most chordates develop, the nerve cord enlarges and becomes the central nervous system.

- The front end of the nerve cord enlarges and becomes the brain.

- The remainder of the nerve cord becomes the spinal cord.

nerve cord

nerve cord

Pharyngeal PouchesPharyngeal pouches are sac-like projections found in the region between the mouth and digestive tube.

- Many chordates have several pairs of pharyngeal pouches.

- Ancient invertebrate chordates such as tunicates and lancets use them for filter feeding.

- In fish pharyngeal pouches become gills

Pharyngeal pouches- In humans pharyngeal pouches

are present only during embryonic development.

VertebratesVertebrates make up the largest group of animals in the chordate phylum. They include jawless fish, jawed cartilaginous fish, bony fish, amphibians, reptiles, birds, and mammals.

Vertebrates contain all chordate characteristics plus …

- endoskeleton made up of bones and cartilage

- backbones (vertebrae) that protects the spinal cord

- skull that protects the brain

Because vertebrates are animals, they also have all the characteristics of animals that we learned earlier.

Ectotherms and EndothermsVertebrates can be divided as ectotherms (cold-blooded) or as endotherms (warm-blooded).

Ectotherms include fish, amphibians, and reptiles.

- Their internal body temperature changes with the temperature of its surroundings.

Endotherms include birds and mammals.

- Their core body temperature is kept nearly constant through thermal homeostasis.

Origin of ChordatesChordates are believed to have descended from echinoderm-like ancestors during the Precambrian period.

Homework

Define the words: chordate, notochord, postanal tail, nerve cord, pharyngeal pouch, endoskeleton, cartilage, vertebrae, ectotherm, endotherm