Class Amphibia
Three orders of amphibians are:
- Urodela - salamanders
- Anura (no tail) - frogs and toads
- Apoda (no feet) - caecelians
Characteristics of amphibians
- Ancestor may be related to the lobe-finned fish
- Circulatory system has a 3 chambered heart with a separate blood circuit through the lungs
- Skin must remain moist. It is highly vascularized because it acts as a respiratory surface (sometimes the only respiratory surface).
- Some amphibians, the caecelians, have internal fertilization. Most amphibians deposit eggs in water where they are externally fertilized.
- All amphibians are at least in part, dependent on environmental water.
Class Reptilia
The reptiles have the following characteristics
- Males possess a penis for efficient copulation and internal fertilization.
- Amniotic eggs - the embryo is surrounded by a special fluid-filled sac known as an amnion, as well as other membranes, which cushion, moisturize, and otherwise protect the embryo.
- Adults prevent water loss by removing nitrogenous wastes as the solid -- uric acid -- rather than a urine solution.
- Water loss is further minimized by protective scales covering a dry skin. The skin's lack of elasticity elasticity requires that it be shed from time to time as the animal grows.
- Reptilian lungs have many smaller compartments increasing the surface area compared to the simple sacs found in amphibians
- Most have a 3 chambered heart, but a partial septum divides the ventricle. Crocodiles and alligators have 4 chambered hearts.
- Dinosaurs, a very successful group of reptiles, went extinct about 65 million years ago. One kind, however, may have survived; those whose scales were modified to become feathers. As you may know, the birds have many similarities to this group of dinosaurs.
Reptiles include snakes, lizards, turtles, crocs and alligators, the tuatara, and dinosaurs.
Class Aves
The characteristics of birds involve modifications for flight as well as those for dry land.
- A light strong skeleton. Many bones are hollow containing an air-filled spongy crisscrossing of supporting elements.
- Teeth have been replaced with a light but tough keratin beak
- The breastbone (sternum) is enlarged with a keel to which the massive flight muscles attach
- The tail is greatly reduced
- Feathers, the hallmark of all birds are modified scales. They take a variety of forms, most being essential for flight.
- All feathers are stiff hollow and highly branched rods.
- Feathers also provide insulation necessary for the high metabolism of endothermy found in birds.
- Feathers are also important in mating and other displays
- The cardiopulmonary system is very efficient with a 4 chambered heart and a very special respiratory system designed for the low P02 at high altitude flight. It allows for the one-way passage of air over a very large and heavily vascularized respiratory surface.
The most famous bird fossil is Archaeopteryx. Considered an example of a missing link archaeopteryx had teeth, a long bony tail and claws protruding from the wings. Regardless of these and other reptilian features, which surprisingly align it with the crocodiles, archaeopteryx had feathers designed for flight.
Look into the controversy over whether birds are really the living relatives of dinosaurs.
Class Mammalia
The characteristics which distinguish the mammals include the following.
- Body covered (insulated) with hair or fur all of in part.
- Glands on the ventral side of the body to provide the young with nourishment (milk). It is the mammary glands for which the class is named.
- Like the birds mammals are endothermic.
- Three of the reptilian jaw bones have been drastically reduced in size and shifted in position to become the bones in the ear.
- A muscular diaphragm which helps move air in and out of the lungs
- Limbs as in birds are rotated down to lift the body off the ground. Such a limb arrangement permits greater efficiency in catching, holding, and killing prey.
- Mammals have socketed teeth, often specialized for the mammals diet, in a fully mobile chewing jaw.
Three orders of mammals are:
- Monotremes - echidna and duckbilled platypus - lay eggs
- Marsupials - a pouch protects the young which must crawl into it and attach to a nipple.
- Placentals - the uterus and placenta allow for full development within the mother