(To complete all classifications ETI has added the Kingdom and the Phyla of all the different taxa treated on this DVD-ROM without higher classification descriptions. Texts from Lynn Margulis and Karlene V. Schwartz, Five Kingdoms. CD-ROM Copyright 2002 ETI / Freeman & Co Publishers)
Members of this phylum, our own, are the most familiar of all the animals. Craniates include about 45,000 species, including most animals of direct economic importance, except molluscs and arthropods. The eight classes of craniate chordates include Cyclostomata (hagfish and lampreys), Chondrichthyes (sharks and other cartilaginous jawed fish), Osteichthyes (bony fish), Choanichthyes (lungfish), Amphibia (toads, frogs, and salamanders), Reptilia (turtles and lizards), Aves (birds), and Mammalia (mammals).
All craniates have a brain that lies within a cranium (skull), which distinguishes members of this phylum from the acraniate chordates—urochordates (Subphylum Urochordata) and cephalochordates (Phylum Cephalochordata). Craniates are grouped by some as Subphylum Vertebrata in Phylum Chordata, together with Subphyla Urochordata and Cephalochordata. The presence of three defining chordate characteristics in craniates suggests that this phylum and the other chordates have common ancestry. In craniates, the first chordate characteristic—the dorsal, single, hollow (fluid-filled) nerve cord—becomes the brain and spinal cord during embryogenesis. The second characteristic defining chordates—a cartilaginous rod called the notochord—forms dorsally to the gut in the early craniate embryo. This slender flexible cylinder of cells containing a gelatinous matrix and sheathed in fibrous tissue extends the length of the body and persists throughout the life of all members of Classes Cyclostomata and Chondrichthyes. In all other classes of craniates, the embryonic notochord is largely replaced by the bony vertebral column—the backbone—during later development. The third chordate feature is the presence of gill slits in the pharynx at some stage of the life history, as in urochordates and cephalochordates (Phyla Urochordata and Cephalochordata). Gill slits reveal the aquatic ancestry of the phylum. These slits are present only in the embryo and larva of land-dwelling craniates with few exceptions, such as the axolotl, a Mexican amphibian. Gill slits persist in adults of aquatic craniates in classes Cyclostomata, Chondrichthyes, Osteichthyes, and Choanichthyea. In terrestrial craniates, gill slits grow shut or are transformed into other structures such as the Eustachian tube and jaws, so gill slits are absent as such in the adult.
Craniates, like all chordates, are bilaterally symmetrical animals that develop from three embryonic germ layers: ectoderm, mesoderm, and endoderm. A well-developed coelom lined by a tissue layer called the peritoneum arises from the embryonic mesoderm. Thin membranes called mesentery suspend the internal organs in this coelom. The bodies of craniates are segmented. The backbone is a series of vertebrae associated with nerves and muscles that are replicated in a series mirroring the segmented muscles present in cephalochordates. All craniates have a digestive tract complete with mouth and anus. Although craniates reproduce sexually, a few species also reproduce parthenogenetically. These species consist of uniparental females in which no fertilization of the egg is required for development of the offspring. In the vast majority of species, male and female are separate individuals. Sexual reproduction requires the fertilization of a comparatively large egg by a much smaller, undulipodiated sperm.
Craniates grouped as agnatha lack jaws and paired appendages. All the other craniates are the gnathostomata, which have jaws—facilitating food getting and defense—and paired appendages. The only class of extant agnatha is Cyclostomata, the lampreys (Petromyzon), hagfish, and slime eels—aquatic animals that have gill slits and a round mouth like a suction cup with horny teeth on a protrusible tongue. The cyclostome axial skeleton, cartilaginous as in sharks, is essentially a reinforced notochord. The skull is a rigid box of cartilage that protects the brain. Cyclostomates have smooth, scaleless skin and lack the scales and fins of cartilaginous fish (such as Squalus) and bony fish. The cyclostomate does not have the ability to regulate its body temperature. The oldest remains of undoubtedly vertebrate animals is that of an ancient armored (jawless) fish with large scales called an ostracoderm, a member of a class of agnathids that is extinct. This class was a transitional form between cyclostomes (jawless fish) and jawed fish. Extant agnathids lack bones.
Of the 25,000 fish species, most—such as Fundulus and Gadus—are bony. Many species of sharks and bony fish are currently endangered by commercial exploitation and destruction of their near-shore nursery areas.
The classes Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish) are both gnathostome fish; they retain gills as adults, have paired fins, have jaws, and lack four limbs. Members of Chondrichthys lack bones; their skeletons and skull are made instead of a softer but more flexible cell product called cartilage. Marine cartilaginous fish include sharks, skates, and sting rays. Their scales, called placoid scales and composed of a dentine plate covered by enamel, differ in origin from the scales of reptiles and bird legs. The extinct jawed, armored fish—placoderms—are the fossil predecessor of modern cartilagenous fish (Chondrichthyes) and bony fish (Osteichthyes).
Lungfish—Class Choanichthyes—live in African freshwater lakes with low dissolved oxygen. When a lake dries up, lungfish secrete a cocoon, dig into the mud, and gulp air into their exposed mouths.
All other fish besides cartilaginous fish and lungfish belong to Class Osteichthyes, the bony fish—salmon, tuna, trout, bass—and most saltwater and freshwater fish. Their scales are thin in comparison with placoid scales and are called cycloid or ctenoid, according to whether the outer edge of the scale is rounded (Greek kyklos, a circle) or toothed (Greek ktenos, comb). Moreover, cycloid and ctenoid scales form internally in the skin and do not emerge from the skin as do the placoid scales of the shark. The heart of a fish pumps blood from the body through a single atrium and a single ventricle, and then to the gills, from which oxygenated blood returns to the body; the circulation is closed.
The remaining classes of gnathostomes, Amphibia, Reptilia, Aves, and Mammalia, are all tetrapods (Greek tetra, four; podos, foot), having four limbs (except for some reptiles). Modern tetrapods are the most visible animals in terrestrial habitats today. We can trace the evolution of tetrapods by means of fossils and the only extant lobe-finned fish—the coelocanth—to the primitive ancestors that link extinct lobe-finned fishes to their descendants—to amphibians, to reptiles, and subsequently to birds and mammals. The bony fishes and amphibians have given rise to no other class of animals.
Members of Class Amphibia, about 200 described species, including salamanders (Salamandra), frogs (Xenopus), and toads (Bufo), lack scales and respire through their moist, flexible, scaleless skin and across the moist mouth lining, through gills—externally visible in some species—or through relatively small lungs. Some salamander species lack lungs and thus respire entirely across their moist skins. The amphibian heart has two atria but a single ventricle. Many but not all amphibians lay eggs and release sperm in water, where fertilization takes place. Most amphibians spend at least the earlier part of their lives in larval or juvenile form as aquatic beings; a few tropical frogs lack larvae—their embryos develop directly into miniature adults carried on the parent’s body or in the minipond formed by a bromeliad. Some amphibians, such as the leopard frog, have aquatic larvae with gill slits (tadpoles). Bullfrogs and leopard frogs live much of their adult lives in fresh water. The Mexican axolotl retains its gilled larval form even as a sexually mature, aquatic adult.
Members of Class Reptilia have dry skin covered with scales—affording protection from desiccation and predation. The tissue layer from which the scales of reptiles develop differs from the tissue layer from which scales of fish develop. Reptiles develop from an egg more adapted to life on land—they are fertilized internally, have a leathery shell, and are retained in the mother until they hatch, in some species—than are the eggs of amphibians. Like embryos of birds, the gilled reptile embryo develops in its own small sea—a fluid-filled compartment enclosed by its eggshell. Reptiles lack aquatic larvae. Turtles (Chrysemys), lizards, snakes (Crotalus), crocodiles, and alligators together total about 5000 species. The popular Mesozoic era dinosaurs belonged to this class. Living reptiles are poikilothermic (unable to internally regulate body temprature): some species are adapted to a wide range of internal body temperatures; others regulate their temperature with behavioral adaptations such as basking in the sun or taking shelter from direct sun by moving to cooler, moister microhabitats. The leatherback turtle regulates its internal temperature somewhat. Each reptile tooth is generally rather similar to the others. Reptiles breathe by using lungs, although their scaly skin is slightly permeable to gas. The reptilian heart has two atria, and the ventricles are not completely separated (permitting partial mixing of oxygenated and deoxygenated blood), except in crocodiles, which have a complete septum between the left and the right ventricles. Most reptiles are well adapted to terrestrial life. Several sea turtle and snake species are endangered—overharvested for meat, shell, eggs, and skins. Baby sea turtles hatch from eggs laid on land but are disoriented by artificial night lights along the ocean, which they must reach to feed and breed. Floridians and other coastal residents are urged to turn down outdoor night lights to allow turtle hatchlings to reach the ocean.
Feathered reptiles—that is, birds—are placed in their own class, Aves. Nearly 9000 living bird species—among them, Phasianus, Podiceps, Puffinus, and Rhea—are recognized. To understand avian structure and behavior, it is best to think of birds as feathered reptiles derived from a branch of dinosaur reptiles, the ornithischians; ornithischian hip bones resemble those of contemporary birds. Aves have land-adapted eggs with porous calcium carbonate shells and internal membranes that facilitate gas exchange. Bird embryos deposit insoluble nitrogenous waste during their life within the shell; bird eggs develop externally. The forelimbs of many but not all (Struthio) bird species are modified for flight as wings, and their bones are hollow. Their scaly skin is studded with feathers; bird scales (look at the leg of a bird) are modified reptilian scales. The saying “scarce as hen’s teeth” is based on biological fact: birds lack teeth. Birds have a four-chambered heart with two atria and two ventricles, and—like mammals—they regulate their internal temperature within a few degrees. Animals with this ability are called homeotherms. Gavia, the loon, has a lower body temperature than that of most other bird species.
Class Mammalia contains about 4500 living species, including Homo sapiens, Bos, Felis, Phoca, Rangifer, and Sus. As mammals, we nourish our young with milk, the nutritious and immunoprotective secretions of the mammary glands of the mother. All mammals are homeotherms—regulators of body temperature. Some species, such as our own, allow only a small range of variation. Others—opossums and hibernators such as woodchucks and ground squirrels—evolved a much broader range of body temperatures. The hair that covers the skin of many mammals at some stage of life is one of several physical and behavioral temperature-control features. The mammalian heart has four chambers. Mammals have complete double circulation: the oxygen-rich blood in the arteries does not mix with the oxygen-depleted blood in the veins. In most mammalian species, an elaborate organ, the placenta, nourishes the developing embryo; fertilized eggs develop inside the female and young are born live. Parental care, although not absent in other classes of vertebrates and even some invertebrates, is well developed in many mammals. Mammals have complex and differentiated teeth.
There are nearly twenty orders of mammals in two great subclasses, Prototheria and Theria. Subclass Prototheria includes all the egg-laying mammals of Australia, New Guinea, and Tasmania. Subclass Theria—all mammals that do not lay eggs—includes all other mammals. The spiny anteater, Tachyglossus aculeatus, and the duck-billed platypus, Ornithorhynchus anatinus—both prototherians—have lower body temperatures than do most other mammals (28.3°C compared with 38°C). The cloaca of prototherians, like that of birds and reptiles, is a common chamber for digestive waste, excretory products, and eggs or sperm. Both prototherian genera also have horny beaks or bills but lack true teeth. They lay shelled and yolk-rich cherry-sized eggs, have an external pouch, and reptilelike bones. All egg-laying mammals nourish their young with milk from primitive mammary glands after hatching.
Subclass Theria comprises two infraclasses—Metatheria (marsupials such as opposums and Phascolarctus, the koala) and Eutheria (placental mammals—Balaenoptera, Cavia, Pan, Vulpes). The young of kangaroos and other metatherians are extremely immature at birth after a brief sojourn in the mother’s uterus. Their relatively well developed forepaws permit metatherian newborns to crawl into an exterior pouch in which young suckle milk while attached to the mammary glands as they continue development. The metatherian female has a cloaca, two vaginas, and a double (Y-shaped) uterus. The eutherian female has a single uterus and single vagina. Eutherian young undergo considerable development inside the mother’s uterus, where they are nourished inside her body by the transfer of nutrients and by gas exchange through the placenta before she gives birth. Eutherian orders include Insectivora (hedgehogs, shrews, moles), Primates (lemurs, tarsiers, monkeys, apes, humans, chimpanzees, gorillas), Hyracoidea (hyraxes), Chiroptera (bats), Dermoptera (flying lemurs), Rodentia (porcupines, mice, squirrels, chipmunks, capybaras), Carnivora (dogs, cats, bears, otters), Pinnipedia (seals, sea lions), Tupaioidea (tree shrews), Edentata (sloths, armadillos, anteaters), Pholidota (pangolins), Lagomorpha (hares, rabbits), Cetacea (whales, dolphins), Tubulidentata (aardvark), Proboscidea (elephants), Sirenia (sea cows, manatees) Perissodactyla (zebras, horses, tapirs), and Artiodactyla (pigs, camels, llamas, deer, cattle, bison).