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Chapter 42 Amphibians 42-1 Origin and Evolution Amphibian comes from the Greek for “double” and “life” Evolved from lobe-finned bony fishes 370 million years ago Became land the first vertebrates to live on Adaptation to Land Hypothesis? Drying up pools vs. limited food and space Left water to escape predation and competition Numerous species of fish in water Terrestrial invertebrates became new food supply Early Amphibians Ichthyostega: earliest known fossil 4 strong limbs Homologous to pectoral/pelvic fins Primitive lungs Overall fish-like body Organs for detecting scents and sounds in air Large, sharp teeth Diversification 3 Orders Anura: frogs and toads Urodela: salamanders Apoda: caecilians Modern Amphibians Key characteristics Metamorphosis Change from aquatic larval stage to terrestrial adult form Moist, thin skin w/ no scales Feet lack claws and are often webbed Use gills, lungs, and skin in respiration Eggs lack shells Order Anura Anuran means “tailless” Frogs and toads Many spend at least part of their life in water Body adapted for jumping Adults are carnivores Smooth moist skin vs. rough bumpy skin Some have sticky tongue that is extended Many return to water to reproduce External fertilization Order Urodela Salamanders Elongated body, long tail, moist skin Range in size (a few cm to 1.5 m) Habitat: Most live in N. America & Central America No species found in Australia Reproduction Lay eggs in water = swimming larva Lay eggs in moist land env. = hatch into mini adult Internal fertilization Females pick up sperm packets Order Apoda Common name Caecilian Live in tropical areas Specialized group of legless amphibians Often blind Resemble small snakes Small eyes located under skin or bone Rarely seen Detect prey through a chemosensory tentacle located on the side of the head Internal fertilization 42-2 Characteristics of Amphibians External Covering Skin serves 2 functions Respiration and protection Moist and permeable to gases and water Mucous glands supply lubricant Also can contain glands that secrete foul-tasting or poisonous substances Make amphibians vulnerable to dehydration Internal anatomy Terrestrial vertebrates must rely on internal skeleton for support Vertebrae of spine interlock Cervical vert. allow neck movement Strong limbs Connect to pectoral and pelvic girdles Specializations for absorbing force Fused bones: radio-ulna & tibiofibula Heart and Circulatory System Divided into 2 loops Pulmonary circulation Carrys deox. blood from heart to lungs, returns ox. Blood back to heart Systemic circulation Carries ox. blood from heart to muscles and organs, brings deox. Blood back to heart Advantage (fish) over single loop Faster blood flow Respiration Larval amphibians (tadpoles) respire through gills and skin Most adults lose gills during metamorphosis Respire through lungs= pulmonary respiration Use positive pressure breathing Diagram page 826 Respire through skin= cutaneous respiration Digestive system Elastic esophagus and stomach allows amphibian to swallow large amounts of food Glands in stomach secrete gastric juices for breakdown Pyloric sphincter relaxes and food moves to small intestine Small intestine Duodenum (upper portion) Ileum (coiled middle portion) Mesentery (membrane like plastic wrap that holds small intestine in place) Digestion is completed and nutrients released into body Digestion system cont’d Large intestine Indigestible wastes are collected Pushed by muscle cavity into Cloaca Cavity also collects from kidneys, bladder, eggs, sperm Waste exit the body through vent Excretory system Kidneys: Filter nitrogenous waste from blood Wastes + water = Urine Larvae excrete the wastes as ammonia Very toxic Adults primary excretory organ excrete wastes as urea Uses more energy but conserves water Nervous system Brain about same size as fish Olfactory lobes are larger (sense of smell) Sense organs Lose lateral line system in metamorphosis Smell, sights, hearing more well developed Eyes covered by transparent, movable membrane called nictitating membrane Sound receptors located inner ear Transmitted by the tympanic membrane (eardrum) and the columnella (small bone that connects the two)
