Chordates: Fishes to Mammals: Quick Revision

A brief note on Chordates: Fishes to Mammals. Read it before a test. Try each question at the end before you open its answer.

The phylum

• Four features at some stage: notochord, dorsal hollow nerve cord, paired pharyngeal gill slits, post-anal tail.
• Non-chordates: no notochord, a ventral, solid, double nerve cord, no gill slits, a dorsal heart, no post-anal tail.
• Urochordata: notochord only in the larval tail (Ascidia, Salpa, Doliolum).
• Cephalochordata: notochord from head to tail, for life (Branchiostoma).
• Vertebrata: notochord in the embryo, replaced by a vertebral column.
• Vertebrates add a ventral muscular heart, kidneys and paired fins or limbs.

The seven classes

• Cyclostomata: no jaws, a sucking mouth. Heart 2, cold-blooded.
• Chondrichthyes: cartilage, separate gill slits, no air bladder. Heart 2, cold-blooded.
• Osteichthyes: bone, an operculum, an air bladder. Heart 2, cold-blooded.
• Amphibia: moist skin, a cloaca, a tympanum. Heart 3, cold-blooded.
• Reptilia: dry skin with scales or scutes. Heart 3 (4 in crocodiles), cold-blooded.
• Aves: feathers, pneumatic bones, air sacs. Heart 4, warm-blooded.
• Mammalia: hair, mammary glands, pinnae. Heart 4, warm-blooded.

• Cartilaginous fishes: placoid scales, claspers, internal fertilisation, often viviparous.
• Bony fishes: four pairs of gills, external fertilisation, mostly oviparous.
• Amphibians breathe with gills, lungs and skin; development goes through a larva.
• Ornithorhynchus is the egg-laying mammal.

A worked example

Name the class of an animal with hair, a four-chambered heart and wings.

1. Hair is found only in mammals.
2. Wings are not enough to make a bird: a bird must have feathers.
3. Class: Mammalia. The flying fox, Pteropus, fits.

Practice with new numbers

A program to try

The program prints one line for each class of vertebrates: its mark, its heart and its blood.

Mistakes to avoid

• Calling every chordate a vertebrate. Sea squirts and lancelets have no backbone.
• Giving a shark an operculum. Its gill slits open separately.
• Linking four chambers with warm blood. Crocodiles have four and are cold-blooded.
• Saying every mammal gives birth to live young. The platypus lays eggs.

Check yourself

1. In which subphylum is the notochord lost when the adult forms?

2. Give two features that set a bony fish apart from a cartilaginous one.

3. Which two classes of vertebrates are homoiothermous?

4. Name an amphibian with no limbs.