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Lecture 18

Lecture 18 - Amphibians.docx

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McGill University
Biology (Sci)
BIOL 111
Suzanne Gray

Lecture 18 - Amphibians Why move on land? Devonian droughts (~400 mya)  Shallow inland seas, swamps, ponds  Low dissolved O2  Increased competition in water o Crowding in small pools  New food resources on land o Example: arthropods and plants  No vertebrate predators on land Difference between land and water as relevant to life:  Availability of water/moisture  Density of medium  Amount of oxygen  Stability of temperature  Amount of UV radiation Clicker Question: All are problems to be encountered on land EXCEPT  amount of oxygen (far more oxygen on land than in water) Problems on land: 1. Water needed to prevent desiccation\  Gas exchange requires a moist surface  Most require water for fertilization and larval development 2. Air is less dense than water  Require stronger skeletal support, muscles  Require more energy, more oxygen brought in and distributed 3. Air temperature is more variable  Body temperature will fluctuate more  Need to modify behavior or physiology 4. UV radiation more intense on land  Need physical protection or change behavior Advantages of Terrestrial respiration:  Air has higher concentration of oxygen than water  Gases diffuse faster in air than water Lung Evolution:  Lobe-finned fishes: 2 ventral pockets formed off esophagus o Used as a supplemental respiratory device  Ray-finned fishes: single dorsal pocket formed off esophagus o Used as a supplementary respiratory device Clicker Question: What is the embryonic origin of the gas bladder? Endoderm, gas bladders are pockets off the digestive tract. Lecture 18 - Amphibians Early Tetrapods:  Stronger limbs and girdles, vertebral column, ribs  Tail used for balance, not swimming  Lungs were primary respiratory organ  External and internal nostrils (nares) Tetrapod solution to air being less dense than water: 1. Stronger limbs, vertebral column, ribs 2. More efficient circulatory system  3+ chambered heart  Double circuit of blood flow Respiratory System:  Function: exchange gases of cellular respiration  Types: diffusion across all body
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