A rohu, a frog, a cobra, a pigeon and a bat all have a backbone. So what separates them? Follow three threads through the groups: skin covering, heart chambers and body heat. Most exam questions on vertebrates are built from these three.
Chapter 1 of the PECTAA Biology 11 book is Biodiversity and Classification. Section 1.5 sorts vertebrates into five groups. This note is for FSc Part 1 (1st year, Class 11) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.
What to know
Vertebrates have a vertebral column and a cranium. The book describes them in five groups.
1. Pisces (fishes). Aquatic, in fresh or sea water. The body is streamlined (fusiform) and covered with scales such as placoid, cycloid or ctenoid. Breathing is through gills. The heart has two chambers, one auricle and one ventricle, and circulation is single and incomplete. Paired and unpaired fins move and balance the body. Most lay eggs, though some give birth to young. Fertilisation is usually external in bony fishes and internal in cartilaginous fishes. Fishes are ectothermic, so body temperature follows the surroundings.
The book's table splits fishes into:
- Jawless fishes: class Myxini (hagfish) and class Cephalaspidomorphi (lamprey, with a sucking mouth).
- Jawed fishes: class Chondrichthyes (cartilaginous skeleton, no swim bladder or lungs; sharks, skates, ratfish) and class Osteichthyes (bony skeleton; lungfish, coelacanths).
2. Class Amphibia. Unlike fishes, amphibians have a neck: the first vertebra lets the skull move up and down. The skin is smooth, moist and without scales, and helps in gas exchange. The heart has three chambers: two atria and one ventricle. Larvae breathe by gills, adults by lungs and skin. Fertilisation is usually external. Examples: salamanders and newts (tailed), frogs and toads (tail-less), caecilians (legless).
3. Class Reptilia. The first group with the amniotic egg. Its three membranes, amnion, allantois and chorion, protect the embryo from drying out, feed it and let it develop on land. The egg holds plenty of yolk and albumin inside a leathery shell. Skin is dry and scaly. The first two neck vertebrae, atlas and axis, give more head movement. The ventricle is incompletely divided. Reptiles are ectothermic. Examples: lizards, snakes, tuatara, crocodiles.
4. Class Aves (birds). Feathers give lift, steering and insulation. Birds are endothermic and keep a nearly constant core temperature, which makes them homeotherms. Forelimbs are wings, bones are light with air spaces, a bill replaces teeth, and a keel on the sternum holds flight muscles. A crop stores food and a gizzard crushes it. The heart has four chambers. The voice organ is the syrinx, and air sacs connect to lungs made of parabronchi. Fertilisation is internal. Running birds such as the ostrich and kiwi have lost flight.
5. Class Mammalia. Milk from mammary glands and hair on the body define the group. Sebaceous glands give oil and sweat glands help cooling. Most have milk teeth and then permanent teeth. There is an external ear (pinna) and three middle-ear bones: malleus, incus and stapes. Mammals are endothermic with a four-chambered heart and internal fertilisation.
Three groups of mammals.
- Monotremes lay eggs: duckbill platypus, spiny anteater (echidna).
- Marsupials carry immature young in a pouch: kangaroo, opossum.
- Placentals nourish the foetus through a placenta and give birth to young: dolphin, rat, monkey, bat, elephant, human.
Key points
- Heart chambers climb: 2 (fishes), 3 (amphibians), partly divided ventricle (reptiles), 4 (birds and mammals).
- Body heat: fishes, amphibians and reptiles are ectotherms; birds and mammals are endotherms.
- Land eggs: the amniotic egg starts with reptiles and is also found in birds and mammals.
Worked examples
Example 1. Why can reptiles breed away from water when amphibians usually cannot? Because reptile eggs are amniotic. The membranes stop the embryo drying out and supply food, and the shell lets gases through but not water.
Example 2. A student says "all mammals give birth to live young". Correct it. Monotremes are mammals that lay eggs. The book names the duckbill platypus and the echidna.
Example 3. Place a shark and a lungfish in their classes. Shark: Chondrichthyes (cartilage skeleton). Lungfish: Osteichthyes (bony skeleton).
Common mistakes in the exam
- Giving amphibians a four-chambered heart. They have three: two atria, one ventricle.
- Calling birds ectothermic. Birds make their own heat and keep it nearly constant.
- Mixing larynx and syrinx. Birds have a syrinx at the lower end of the trachea; mammals have a larynx with vocal cords.
- Putting the kangaroo with placentals. It is a marsupial.
- Listing the amniotic egg under amphibians. It begins with reptiles.
Exam technique
- In "distinguish" questions, compare the same features side by side: skin, heart, breathing, body heat, fertilisation.
- Give one book example for each group you name.
- Draw and label the amniotic egg if a diagram is asked: amnion, allantois, chorion, yolk sac, shell.
- Check your board's notice for the topics in your paper.
Practice questions
- Name the three membranes of the amniotic egg. Answer: Amnion, allantois and chorion.
- How many chambers does the fish heart have? Answer: Two: one auricle and one ventricle.
- What is the function of the keel in birds? Answer: It gives attachment to the flight muscles.
- Name the three middle-ear bones of mammals. Answer: Malleus, incus and stapes.
- To which mammal group does the echidna belong? Answer: Monotremes, the egg-laying mammals.
Ready to practise this? Continue in 1st Year Biology.