The study of some of the different vertebrate animals gives us an understanding of how animals have adapted to their environments. Comparing anatomic differences between animals, let's understand how they are able to function in the habitats in which they live. In the Habitat Land of Frog Island, you can see a few examples from among the over 3800 species of frogs, some making minor changes that let them live in trees, or others to live as toads in the desert, while most live at least a part of their life in water.

To help everyone 'get on the same page', by knowing what animals belong in the same groups, scientists led by Aristotle, about 350B.C., and Linnaeus during the 1700's, have been classifying them for centuries, as new animals are found, or new things are learned about them. A scientific name is given, often in Latin or Greek, and a common name is used. The same is done for all living things. The classifying names given, from the most general category, to the most specific one, are: KINGDOM, PHYLUM, CLASS, ORDER, FAMILY, GENUS, AND SPECIES. Most scientists like the five-Kingdom system that divides the hundreds of thousands of living things into five Kingdoms: Monera (simple cells without nuclei), Protista ( single cells with nuclei), Fungi (many cells, depending on food from outside themselves), Plants (multicellular, making their own food with photosyntheses), and Animals (multicellular, depending on other sources for food.) The organisms of the animal kingdom, divided into nine Phylums, (pl., phyla), includes sponges, worms, insects, oysters, etc, but the most advanced group that we're interested in here, the vertebrates, are a Sub-phylum of the Phylum: Chordata.

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Vertebrate animals live in the sea, on land, and in the air! Two features about their anatomy, their vertebral column, and their cranium (head), make them different from the others. There are three main Classes of Fish, with about 20,000 species, and four Classes of tetrapods, with over 23,000 species, for a total of over 43,000 species in Phylum: Chordata. The tetrapods (having four extremities, or feet (or hands)) include the Classes: Amphibians, about 4,000 species, Reptiles, about 6,000 species, Aves (birds), about 8,600 species, and Mammals, about 4,500 species. Comparisons of these Classes of animals is an important way we learn about our World; dinosaurs were early reptiles, but some could fly, too. Crocodiles, alligators, and caimans are the dinosaur's modern reptilian survivors, penguins are birds, and humans (Homo sapiens) are mammals.

Most of the animals that live in or around water, the fish, amphibians and reptiles, have body temperatures like their environment; we call them ectothermic animals, meaning controlled by the external, or outside, heat. But birds and mammals regulate their own temperature, so we call them endothermic animals.

Another interesting feature about animal development is that Reptilian, Avian and Mammalian classes of vertebrates live mainly on land, but they all keep a short phase of their early development in a sac of fluid, inside the amniotic sac. That's why we call them amniotes, and call the Fishes and the Amphibians, anamniotes (meaning without the sac of water.) Anamniotes don't need an amniotic sac, because they reproduce in water, mainly in ponds, lakes, streams, estuaries, and in the ocean!

On Frog Island, we find only frogs, one of the members of the Class: Amphibians. This class is divided into three Orders, one for frogs and toads, (Order: Salientia), one contains salamanders, (Order: Urodela), and another, caecilians (Order:Gymnophiona), a worm-like animal that has no feet. Click on Habitat Land of the island, where you'll find several different types of frogs and toads.

The different design patterns of vertebrates are important for understanding how animals adapt to their environments. The basic patterns found among the various orders of fish are repeated, with modification, in adult reptiles, birds, and mammals. However, every genus of animal in the vertebrate sub-phylum uses the same basic pattern at some phase of it's embryonic development. This fact has led to the famous phrase, "ontogeny recapitulates phylogeny". Branchial arches and gill slits are identifiable during the third and fourth weeks of human embryogenesis. We have chosen to emphasize the designs of the respiratory and circulatory systems as examples of the value of studying comparative anatomy. Other organ systems have similar adaptations, perhaps less prominent.

RESPIRATION

One of the most interesting features of vertebrate animals is the way that their anatomy has adapted to get oxygen, an essential 'element of life', from their water or air environments. Oxygen, an element in the Periodic Table of Elements, is a gas that is only slightly soluble in water. Water contains a variable but small percentage of oxygen, depending on its' partial pressure, the temperature, mixing, and presence of other substances, and air has about 21 percent oxygen. That means that animals that live in water must have a very efficient external respiration for removing the small amounts of oxygen to meet their metabolic needs. All animals use oxygen they obtain from the environment to nourish body tissues in chemical processes called internal respiration.

Fish with internal gills collect oxygen using two important features: first, they pass a large amount of water through slits in the back of the mouth which connect it with the skin; these are called pharyngeal slits. The flow of water is one-way, into the mouth and out through the pharyngeal slits. Second, branchial arches, bones that give structure to the slits, allow them to open and close as a two-phase respiratory pump, causing the water to cross the gill curtain, ventilating it. Some fish, like tuna, are in continual movement to keep the water flowing. But most fish fill the mouth with water, keeping the gillslits closed for a moment, to allow for exchange of gasses and chemical wastes; then, they open the slits, releasing the water, and filling the mouth again. This is a 'dual pump' type of flow. The gill curtain contains a thinly-covered, very dense set of capillaries, part of the circulatory system, that perfuses, or brings blood through the curtain. Oxygen crosses into the blood, and wastes, mainly carbon dioxide, wash out. Some amphibians have external gills on the outside of the head that water flows across. This arrangement hasn't been very safe, so the numbers of fish and amphibians with external gills are rather small, compared to those with internal gills. There are many more species of Fish living today, than any other Class of vertebrates, so this gill system of ventilation has been very successful! All vertebrates continue to have internal, gillslit-like structures during early phases of development while they are in a water environment.

Because of this simple, 'dual pump' type of respiratory system in most fish, they are limited to always living in water. The temporary ability for ventilation away from water is possible by using gas bladders, or lungs where oxygen is absorbed. In lungfish, the flow of gas goes in both directions, in, and then out of the mouth; this is called bidirectional or tidal flow, resulting from the action of a 'pulse pump'. Frogs drop the throat to fill the mouth with air, and then, they release the spent air through the nares (nose); then, after closing the nares, they elevate the throat, and to force the air into the lungs. Finally, they flush the mouth by pumping the throat, before starting the cycle again. Many Amphibians also use the skin for part of the external respiration.

Another method, aspiration, the ability to suck air into the lungs, and to force it out, is introduced in reptiles, birds, and mammals. This 'aspiration pump' is made possible by the arrangement of a ribcage, and muscles that expand it, pulling air into the sacs of the lung. The ribcage is part of the axial skeletal system, and the intercostal muscles and diaphragm are part of the skeletal muscles. This method is a major reason that these classes of animals are able to be mobile in their environments. Turtles have trouble with this system because of their rigid exoskeleton, so when submerged, they absorb oxygen from water in the throat. But birds have the most efficient ventilation system of all! They fly at high altitudes without difficulty. Their anatomical arrangement functions like a bellows, in which air is pulled into an extensive set of air capillaries along side of blood capillaries, where the exchange of gasses occurs, and the air already spent of oxygen, and loaded with carbon dioxide, is pushed out through a set of air sacs. Some of these are inside the bones, to add bouyancy, but unfortunately, also the fragility of bird skeletons.

Mammals, even porpises and whales, also use the 'aspiration pump' system, producing a tidal flow with well developed lungs, and many air sacs, called alveoli. Whales have in addition, a huge spermaceti organ in the head, a mass of oils, liquid at warm temperatures, and a solid mass at cooler temperatures, through which they control their bouyancy. Diving for food requires that they increase density by cooling the oils. By shifting a part of their blood through nasal capillaries next to cold seawater, or away from these vessels, they control the spermaceti organ, coordinating with the need to surface for air, blowing out the spent gas, and inhaling fresh air through the blowhole.

CIRCULATION

Similar to the simple, 'dual pump' type of respiratory system, the circulatory system of most fish is also comparatively simple. The heart, made up of four chambers, all in a series, circulates blood in a single circulation pattern. The flow is through a ventral aorta which sends off six paired, aortic arches into gill capillaries, and onward, after reforming into a dorsal aorta, to the somatic and visceral organs of the body. This means that blood flows through the heart only once as it completes its' route. In vertebrates with lungs, a double circulatory pattern is necessary to allow for a more complex respiration system, the lungs. This means that blood goes through the heart twice, in a parallel pattern, before it completes its' route; once through the lungs in the pulmonary circulation, and once through the systemic circulation to the rest of the body. The result is a circulatory system that is variable between, and within the Classes of animals.

Aortic arches have developed specialized patterns of flow, for example, birds have a right-sided systemic arch, from Aortic Arch IV and the dorsal aorta, but mammals developed a left-sided systemic arch from this arch; the ventral aorta is not continued as an important vessel in birds and mammals. Aortic Arch I becomes non-functional among adult fish, and Arch II becomes the internal carotid arteries, and Arches III and IV become the common carotid arteries. In mammals, Arches IV and V make up the main systemic arteries, and Arch VI contributes the pulmonary arteries. Various intermediate configurations of this general theme are apparent in amphibians, reptiles, and birds.

The hearts, too, are quite variable from Class to Class. The basic pattern of fish is a series of four, pulsating chambers in an S-shaped row, with a primitive valve, the atrioventricular valves, dividing the middle two chambers, the atrium and the ventricle. The first chamber, the sinus venosis, a poorly-muscularized bulb above the atrium is filled partly by gravity, and is separated from the atrium by a sino-atrial valve. Some conal valves line the fourth chamber, the conus arteriosis, preventing return after a contraction pulse. As the heart empties, venous blood is aspirated into the sinus venosis and atrium, filling them for another pulse. The heart is enclosed within a snugly-fitting, fluid-lubricated, pericardial sac that confines the pulsating organ, helping to prevent backflow into the thin sinus venosis. In many fish, the sac rests against cartilage or bones that add to the rigidity, and the propulsion of the blood. The sinus venosis is the site of a small sino-atrial node, or cardiac pacemaker, composed of neuron-like fibers, called Purkinje fibers, that are specialized cardiac muscle cells. These cells respond to the autonomic nervous system and the endocrine system. They also respond to filling of the sinus which activates an electrical signal that spreads over the muscle, creating contractions, and blood flow. The response to the stretch of the sinus by filling is called the Starling reflex.

This basic design pattern continues in lungfish, however they have developed a small septum that divides the atrium into a larger, right side and a smaller left side, and an incomplete interventricular septum that divides the ventricles. These incomplete structures act as boundrys for seperating oxygenated and deoxygenated blood, like the fences in a corral guide cattle.

In frogs and other amphibians, the upper septum is complete, but the ventricle has no septum. The flow of oxygenated blood is contained by using deep recesses in the muscle, with vertical walls, called trabeculations, that shunt the blood into the pulmonary and the systemic circulations.

In reptiles, which are much freer to move in terresterial environments, and use more energy that requires more oxygen transport, the sinus venosis is smaller, but the conus arteriosus, the outflow chamber develops a muscular ridge , the beginning of an interventricular septum, and three major vessels, the pulmonary, and the right and left aortic trunks. A connection between the pulmonary and the left aortic trunks, called the Foramen of Pinizza, allows diving reptiles, like croccodiles, to reduce the amount of flow to the lungs. In fact most reptiles also use this feature to slow their respiration rate on land, making them appear, on quick inspection, to not be breathing.

Birds and mammals have four heart chambers, but the sinus venosis, still present in birds, is almost gone in mammals, where the sino-atrial node that coordinates cardiac contractions, is the only recognizable part. The former fourth chamber, the conus arteriosus, is only found in embryos, because it gives rise to the pulmonary trunk and the single systemic aortic arch. Remarkably, as mentioned above, birds develop the right aortic arch and mammals, the left. An animation follows that gives a concept of the structural adaptations of the vertebrate heart from the basic S-shaped tubular, uni-ventricular model of fish hearts, to the four chambered, bi-ventricular heart of humans.

Developing Embryos Are Like Diving Birds and Mammals

Diving birds and mammals have less flexibility for adjusting to breath-holding when they are underwater. Animals that are able to dive successfully respond in three ways: first, they slow the rate of the heart beat by reducing the blood flow to the lungs. Second, they use anaerobic metabolism to use less oxygen, and third, they shunt blood from unused muscles and the digestive system, to essential organs, primarily to the brain and adrenal glands. In the same ways, the embryonic stages of development of amniotes, occuring in the fluid environment of eggs, or in mammals with placentas and an amniotic sac, these diving adaptations are used for protecting the fetus, ensuring successful reproduction.

So, the remarkable adaptation of vertebrate animals, capable of living in the sea, on land, or in the air have developed different, but related anatomic and physiologic systems that appear as a continuum among adults of these Classes. Equally remarkable, the entire group of vertebrates also utilizes at some time during embryonic life, part or all of the basic structural designs and functional features of adults. These facts alone justify having a Comparative Anatomy and Physiology hut on Frog Island.