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Eight Different Classes of Vertebrates Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 29, 2011

Class Osteichthyes: Fish With Skeletons Of Bone

The Class Osteichthyes involves “fishes” (ichthy) whose skeletons are composed of “bone” (oste). This familiar Class of bony fishes (approximately 30,000 known species) is the largest group of existing vertebrates. Most of those we frequently encounter (such as perch, bass, trout, and tuna) are classified as the ray-finned fishes, due to the flexible ribbing of rays or ridges visible within their fins (see Figure 12.5).

There are several other unique features occurring in bony fishes:

1. They have a slimy skin. Coated with a secretion of mucus ( MEW -kus) or “slime,” the bony fishes can easily glide through the water, suffering very little friction as they move.

2. Their gills have an operculum (oh- PER -kyuh-lum) – an external “covering or lid” (opercul). Bony fish are able to breathe while they remain still in the water, because they take oxygen-containing water in through their mouth, and then push it out between their gills, due to the flapping movements of the operculum.

3. Their bodies contain a swim bladder. Unlike sharks, bony fishes have a swim bladder that they can inflate with air, allowing them to remain nearly motionless in the water without sinking. (Sharks, in contrast, have to keep constantly swimming in order to keep themselves from sinking!)

The Chordata: Animals with a “Chord” in Their “Back” Wide Diversity in Backboned Creatures: Eight Different Classes of Vertebrates Class Osteichthyes: Fish With Skeletons Of Bone

Fig. 12.5 Some important features of bony fish anatomy.

Class Amphibia and Reptilia

Class Amphibia: Livers Of A “double Life”

After the first fish appeared in the waters of the Paleozoic (“Ancient Life”) Era, it was not very long before they were joined by the first amphibians. Unlike fish, which only have fins, amphibians are classified as tetrapods ( TET -ruh-pods) – animals with “four” ( tetra -) “feet” (pods).

Between about 350 and 400 million years ago, the Fossil Record provides evidence that there were tetrapod fishes – fish that had four foot-like attachments to their skeletons. It is from these tetrapod fishes that the first amphibians likely evolved, crawling out as pioneers from one life in the water to another on the land. Hence, the word, amphibian, literally means “liver of a double life.”

The main members of the Class Amphibia are the salamanders, frogs, toads, and newts. Perhaps most reflective of the “double life” of the amphibians is the life cycle of a frog. Being hatched from eggs in the water, the frog starts out life as a legless tadpole, propelled by a muscular tail. And, like a fish, it has internal gills. As time passes, however, a dramatic metamorphosis occurs. The tail and gills are progressively resorbed (ree- SORBD ) – literally “sucked in” ( resorb ) to the body, then disappear. Four legs finally develop, and the tail-less frog form at last hops out onto the land.

Class Reptilia: “crawlers” With Backbones

To be sure, the amphibians do more than their fair share of crawling (as well as hopping around)! But the group as a whole is named for the “double lives” its members lead – on both the land and in the water.

Amphibians have moist skins, not covered by scales. Therefore, they must frequently return to a watery environment, or else suffer the deadly consequences of tissue dehydration ( dee -high- DRAY -shun). This term literally means “the process of” (- tion ) losing “water” ( hydr ) “from” ( de -) the body.

Reptiles, however, are much more adapted to life on dry land. Their skin is covered by scales containing keratin ( CARE -uh-tin) or “horn” ( kerat ) “substance” (- in ). Keratin is a family of tough, waterproof proteins found in the skin, hair, claws, and horns of various animals. Hence, one might well nickname the reptiles as the “ keratinized (car- AT -uh-nized) crawlers”! This group includes the turtles, lizards, alligators, and crocodiles, which all move about by crawling. But it also involves the snakes, which have lost their limbs during the course of evolution, and so are now forced to slither!

Reptiles breathe through lungs rather than gills, and their eggs are encased within rubbery, waterproof shells. And within each of these waterproofed eggs, the reptile embryo undergoes its development inside an amnion ( AM -nee-un), much like a “little lamb.” An amnion is a fluid-filled sac that encloses the embryos of the so-called higher vertebrates – the reptiles, birds, and mammals (Figure 12.6).

The Chordata: Animals with a “Chord” in Their “Back” Wide Diversity in Backboned Creatures: Eight Different Classes of Vertebrates Class Reptilia: “crawlers” With Backbones

Fig. 12.6 The amnion: A “little lamb” around the embryo.

Because their embryos are encased within the amnions of waterproof eggs, they breathe through lungs, and their bodies are covered by keratin-stuffed scales, it is not so surprising that the reptiles followed the amphibians out of the water, and onto the land. They are much more protected against dehydration! Hence, they can stay on the land much longer than amphibians. (Alligators and crocodiles, of course, love the water!)

Recall (Chapter 3) that the Mesozoic Era (200–65 million years ago) was called the Age of Reptiles. The dinosaurs and many other long-extinct species of reptiles flourished and dominated the surface of the planet during this time. In other words, the entire Earth was crowded and swarming with “creepy-crawlers”!

Class Aves and Mammalia

Class Aves: They’re Just “for The Birds!”

The Fossil Record strongly suggests that birds evolved from reptiles. A bird is a homeothermic ( HOH -mee-uh- ther -mik) vertebrate that maintains its body “heat” (therm) about the “same” (horneo), has wings, feathers, two legs, a bill or a beak, and lays eggs. Becoming homeothermic, and thus able to regulate the internal body temperature, really was a big deal for evolution!

Prior to the appearance of the birds, all of the other vertebrates (such as fish, amphibians, and reptiles), were classified as heterothermic ( HET -ur-uh- therm -ik) or poikilothermic ( poy -kih-low- THER -mik). This literally means that they had a “changeable” (poikilo) or “differing” (hetero) body “heat” (therm). That is, before the birds, the existing vertebrates had their body temperatures closely tied to the changes occurring in their surrounding external environment. Since they could not self-regulate their own body temperature, they had to resort to various behaviors (such as seen in a turtle or lizard sunning and warming itself on a rock).

Because of this body temperature limitation, reptiles and amphibians, for instance, are only able to live in fairly warm habitats. After all, you don’t see many snakes or frogs near the Arctic Circle! With the appearance of birds, however, all that soon changed! Birds now dwell in nearly every corner of the world, all the while successfully being homeothermic and regulating their own body temperatures.

Probably one of the first vertebrates to make this dramatic transition from heterothermic (poikilothermic) reptile to homeothermic bird was a strange creature called Archaeopteryx ( ar -kee- AHP -ter-iks). Its odd name, Archaeopteryx, means “ancient” (archeo) “bird or wing” (pteryx). A look at Figure 12.7 clearly shows the reasons. Archaeopteryx had such reptile-like characteristics as sharp teeth in its mouth, forelimbs with claws, and a long tail containing vertebrae. Yet, it still was part bird, because it sported a beak, and showed two wings with feathers. Archaeopteryx lived approximately 150 million years ago, and probably flew over the heads of some dinosaurs!

The Chordata: Animals with a “Chord” in Their “Back” Wide Diversity in Backboned Creatures: Eight Different Classes of Vertebrates Class Aves: They’re Just “for The Birds!”

Fig. 12.7 Archaeopteryx: The ancient bird-reptile.

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