Members of this phylum, our own, are the best known of all the animals. In fact, in the minds of many people, chordate is synonymous with animal. All mammals, birds, amphibians, reptiles, and fish (that is, the vertebrates), along with several obscure groups of animals (including some invertebrates), belong to this phylum. There are about 45,000 species, including all the animals of major economic importance, with perhaps the exception of some arthropods and mollusks.
Chordates can easily be defined by the presence of three features. One is the single, dorsal nerve cord which, in mammals, becomes the brain and spinal cord. A second universal chordate feature is a cartilaginous rod, the notochord, which forms dorsal to the primitive gut in the early embryo. This slender bar of cells contains a gelatinous matrix and is sheathed in fibrous tissue. It extends the length of the body and persists throughout life in some of the invertebrate chordates, such as lancelets and lampreys. In the vertebrates, however, in the course of development it is surrounded and later replaced by the vertebral column. The third chordate characteristic is the presence, at some stage in the life cycle, of gill slits in the pharynx or throat. These gill slits reveal the marine ancestry of the phylum. In the land-dwelling vertebrates, these slits are present only in the embryo; they close or transform so that they are absent in the adult animal.
Chordates are bilaterally symmetrical animals that develop from three embryonic germ layers: endoderm, mesoderm, and ectoderm. Their bodies are segmented, a fact that is revealed in the backbone composed of repeated vertebrae. All chordates have a digestive tract complete with mouth and anus and a well-developed coelom that develops from the embryonic mesoderm layer. The internal organs are suspended in this coelom by thin membranes of tissue called mesentery. All chordates reproduce sexually; a very few can also reproduce parthenogenetically. In the vast majority, sexes are separate and large eggs are fertilized by undulipodiated sperm.
According to most current classifications, there are four subphyla of chordates. Animals of the two acraniate subphyla, Tunicata and Cephalochordata, lack a brain.
Most tunicates are sessile marine animals; only the larva has a notochord and a nerve cord, and the adult secretes a tunic, a tough cellulose sac in which the animal is embedded. There are three classes of tunicates: the Larvacea, which are minute and tadpolelike as adults; the Ascidiacea, which as adults grow a typical tunic; and the Thaliacea, the chain tunicates or salps. The bodies of chain tunicates are barrel shaped and banded by muscles as hoops band a wooden cask. An asexual adult produces a chain of hundreds of buds, which then turn into sexual adults.
The cephalochordates, or lancelets, have a notochord and a nerve cord that persist in the adult and extend the length of the body. They are small, scaleless, fishlike, primitive chordates that belong to only one class, the Leptocardii.
All other chordates are craniates: they have a brain and a skull. There are two subphyla: the Agnatha, which lack jaws and paired appendages, and the Gnathostomata, which have jaws and usually have paired appendages as well. The ostracoderms, ancient armored fish with large scales, are a class of agnathids that is entirely extinct. The only living agnathids make up the class Cyclostomata, fish that lack scales and have a round mouth like a suction cup. Lampreys, hagfish, and slime eels belong to this group.
Gnathostomes, the jawed chordates, belong to either the superclass Pisces (the fish) or the superclass Tetrapoda (animals having four limbs). There are two classes of living Pisces. (The placoderms, ancient jawed fish, are all extinct.) The sharks, skates, and rays belong to the class Chondrichthyes, marine fish whose scales, called placoid, are each made of a plate of dentine covered by enamel—like teeth. Sharks and their relations lack bones; their skeletons are made instead of a softer, more flexible material, cartilage. All the other fish belong to the class Osteichthyes, the bony fish. Their scales, made of bony material, are called cycloid or ctenoid according to whether their outer edge is smooth or spiny. Altogether, there are some 25,000 species of fish—mainly of the bony kind.
Zoologists recognize four classes of tetrapods. The amphibians (class Amphibia) lack scales. They respire both through their moist, soft skin and through gills, lungs, or the mouth lining. They lay eggs in water, where they spend at least their early life. There are about 2000 described species, including the frogs, toads, and salamanders.
The featherless reptiles (class Reptilia) have dry skin covered with scales. They develop from an egg that has internal membranes and is adapted to life on land; it is called the amniote egg. There are about 5000 species, including all the turtles, lizards, snakes, and crocodiles. The famous Mesozoic Era dinosaurs belonged to this class. Living reptiles (but perhaps not some extinct ones) are poikilothermic, or cold blooded—they cannot regulate the temperature of their blood very well. Their teeth are generally quite similar to each other. Reptiles breathe through their lungs. Most reptile species are well-adapted to life on land; several turtle and snake species are marine.
The feathered reptiles, or birds, constitute class Aves. There are nearly 9000 living species. They all have land-adapted eggs with calcium carbonate shells; their forelimbs are modified as wings; they have scaly skin with feathers and lack teeth. They are homeothermic—they can regulate their blood temperature internally.
There are about 4500 living species of class Mammalia, to which we belong. Mammals are homeotherms, with some species better at it than others. They have a four-chambered heart with complete double circulation; that is, the aerated blood of the arteries does not mix with the oxygen-depleted blood of the veins. The skin of most mammals is covered with hair at some stage of life. Mammals nourish their young with milk secretions produced in mammary glands of the mother. The fertilized egg develops inside the female; in most mammals, a special organ, the placenta, nourishes the developing embryo. Mammals have complex and differentiated teeth.
There are nearly 20 orders of mammals in two great subclasses: the subclass Prototheria, which includes the egglaying mammals of Australia, and the subclass Theria, which includes all other mammals. The Duck-billed Platypus and the Spiny Anteater, both prototherians, have a cloaca (a-common channel for digestive, excretory, and reproductive products), a horny beak or bill (no true teeth), shelled and yolky eggs, a pouch, reptilian bones, and poor temperature regulation. Theria consists of two infraclasses—Metatheria (the marsupials) and Eutheria (placental mammals). Most metatherians have an exterior pouch in which the young (born live) suckle for most of their development; they also have a cloaca and a double uterus and vagina. Eutherians have a single vagina; the young undergo considerable development inside the mother before they are born, and are nourished inside her by a special organ called the placenta. Eutherian orders include, among others, Insectivora (hedgehogs, shrews, and moles), Primates (lemurs, tarsiers, monkeys, apes, and human beings), Chiroptera (bats), Rodentia (squirrels, mice, and porcupines), Carnivora (dogs, cats, and bears), and Pinnipedia (seals and sea lions).
From: Margulis and Schwartz (1988)