Transmitted to humans by mosquitoes, the various forms of malaria result in more deaths worldwide than any other parasitic disease. A nitrogenous base, such as cytosine, thymine, or uracil, with a characteristic single-ring structure; one of the components of nucleic acids. Composed of magnetic elements i. Because of the inefficiency of this method of respiration, insects must retain small body sizes. Having a large store of oxygen in the lungs causes some problems.
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Text Structure Exam Wisdom Remember it! Understanding Texts Theme What is Non-fiction? In chordates the main nerve cord is single and lies above the alimentary tract , while in other phyla it is paired and lies below the gut. Cephalochordates and vertebrates are segmented, as are the annelids and their relatives; however, segmentation in the two groups probably evolved independently.
The gill slits and some other features that are common among the hemichordates and the chordates originated before the chordates became a separate group. Hemichordates have no tail above the gut and no mucus-secreting endostyle between the gill slits.
An ancestral chordate, as suggested by the adult lancelet and the tadpole larva of tunicates, had a distinct front and hind end, an anterior mouth, a posterior tail above an anus, unpaired fins, and gill slits that opened directly to the exterior. A free-swimming tunicate larva metamorphoses into an attached, sessile adult with an atrium that surrounds the gills. The atrium of lancelets probably evolved independently. The chordate notochord is a stiff rod with a turgid core and fibrous sheath.
It keeps the animal from shortening when locomotory waves are produced through muscular contraction. The chordate body is supported by fluid in the body cavities. In tunicates, added support is provided by the tunic. Cartilaginous material supports the gills and other body parts of tunicates and cephalochordates. Immature vertebrate skeletons generally consist largely of cartilage , which becomes increasingly bony with age.
The cartilaginous skeletons of sharks and some other vertebrates are thought to have evolved from more highly mineralized ones. In both cephalochordates and vertebrates, muscles used in locomotion are well developed and organized segmentally.
The tail musculature of tunicates is simpler and without clear indications of segmentation. There is at least a small amount of musculature throughout the body of all chordates.
As jaws, limbs, and other body parts have evolved in vertebrates, so have the muscles that operate them. The anterior end of the main nerve cord in chordates is enlarged to form at least the suggestion of a brain, but a brain is well developed only in vertebrates. Tunicate larvae have visual organs sensitive to light and sense organs responsive to the direction of gravity.
Pigment spots and light receptors in the nerve cord of lancelets detect sudden changes in light intensity.
The eyes and other sense organs of vertebrates are more elaborate and complex. The presence in cephalochordates and vertebrates of a nervous system with segmentally repeated nerves arising from the dorsal hollow nerve cord is suggestive of a common ancestry.
The tunicate nervous system does not have the segmentally repeated nerves. The brains of all vertebrates are greatly enlarged and subdivided into functionally specialized regions. Both tunicates and cephalochordates are filter feeders of small particles of food suspended in the water. Beating cilia hairlike cellular extensions on the gill slits draw a current of water into the mouth and through the pharynx, where a sheet of mucus , secreted by the endostyle a glandular organ lying below the two rows of gill slits , filters suspended food particles from the water.
Cilia lining the pharynx move the food-rich sheet of mucus upward over the gill slits, and it is then rolled up and transported to the posterior part of the gut. The water current passes into the atrium and exits through the atrial opening. The difference is that the food consists of somewhat larger particles that have been deposited on the bottom detritus , and, instead of the feeding current being driven by cilia, the pharyngeal musculature pumps water and food particles across the gill slits.
The earliest fishes probably fed on detritus, and a sucking action is retained by their extant representatives lampreys and hagfishes. With the development of jaws, it became possible for the vertebrates to capture and seize larger food items. The lower digestive tract of the primitive chordate is a simple tube with a saclike stomach. There are only indications of the specialized areas and of glandlike structures, such as the liver and pancreas, that occur in vertebrates.
The excretion of wastes and the control of the chemical composition of the internal environment are largely effected by kidneys, although other parts of the body, including the gills, may play an important role.
Tunicates and cephalochordates have a salt content essentially the same as seawater, but vertebrates, even marine species, have body fluids of low salt content, with the exception of hagfishes.
A possible explanation is that the vertebrates evolved in fresh water, but it seems reasonable that hagfishes branched off while still marine and that the freshwater form evolved later. A primitive chordate gill is present in tunicates and cephalochordates, where it serves in both respiration and feeding.
The vertebrate gill may retain some role in feeding, although the current is now produced by the action of muscles, not cilia. The gills became reduced in number in various lineages, and they were strengthened by supporting elements, some of which evolved into jaws. Lungs, already present in fishes, became the main respiratory organs of terrestrial vertebrates. The circulatory system in chordates has a characteristic pattern. In tunicates and vertebrates the blood is propelled by a distinct heart; in cephalochordates, by contraction of the blood vessels.
Unoxygenated blood is driven forward via a vessel called the ventral aorta. It then passes through a series of branchial arteries in the gills, where gas exchange takes place, and the oxygenated blood flows to the body, much of it returning to its origin via a dorsal aorta.
The blood of vertebrates passes through the tissues via tiny vessels called capillaries. In tunicates and cephalochordates, capillaries are absent and the blood passes through spaces in the tissues instead. In vertebrates, endocrine glands those of internal secretion produce hormones that regulate many physiological activities. In tunicates and cephalochordates, organs have been identified that correspond in anatomical position to the pituitary gland of vertebrates, but which hormones, if any, they secrete is uncertain.
In vertebrates, the thyroid gland produces thyroxine, an iodine-containing hormone that helps regulate metabolism.
The thyroid is a modified endostyle, as can be illustrated by larval lampreys in which the thyroid still secretes mucus for use in feeding. The endostyles of lancelets take up iodine and form thyroxine, but the thyroxine formed may not function as a hormone in the lancelets themselves. Tunicates largely rely upon the passive defense afforded by their heavy tunic.
Lancelets move rapidly through the substrate, and their well-developed locomotory apparatus evolved largely to provide a means of escaping predators. Vertebrates have ceased to feed on detritus brought to them by water currents.
They have shifted to consuming larger foodstuffs and to actively locating, pursuing, and subduing what they eat. Many scientists maintain that chordates originated sometime earlier than million years ago; that is, they predate the fossil record. Such early representatives were soft-bodied and therefore left a poor fossil record. The oldest known fossil chordate is Pikaia gracilens , a primitive cephalochordate dated to approximately million years ago.
There is disagreement over whether older animals—such as Yunnanozoon lividum and Haikouella both of which date to million years ago and possess several chordate features —should be considered chordates. An extensive vertebrate fossil record begins about million years ago. Embryological evidence places the phylum Chordata within the deuterostomes bilaterally symmetrical animals with undeterminate cleavage and whose mouth does not arise from the blastopore , which also includes the phyla Hemichordata, Echinodermata, and Chaetognatha.
The closest relatives of the chordates are probably the hemichordates, since these animals possess gill slits and other features not found in other animal phyla. A slightly more remote relationship to the echinoderms is inferred on the basis of resemblances between the larvae in some groups of hemichordates and echinoderms. The derivation of chordates from certain fossil echinoderms has been argued on the basis of features such as what appear to be gill slits.
Theories that derive them from other phyla e. Whether the first ancestral chordate was more like a tunicate or a cephalochordate has been extensively debated. The classical theory is that the ancestor was like a cephalochordate and that one lineage became attached to hard surfaces and evolved into tunicates, whereas another remained unattached and evolved into vertebrates. An alternative theory is that the ancestor was like a tunicate and that the other two subphyla arose by modification of the tadpole larva.
There is some preference for the classical theory because it provides the most satisfactory way of accounting for the similarities between chordates and hemichordates of the subphylum Enteropneusta. Within the chordates, the tunicates probably branched off before the common ancestor of cephalochordates and vertebrates arose, for the latter resemble each other in some details of neuroanatomy and biochemistry. This outline gives the major groups of chordates.
Modern systematic biology attempts to arrange groups of organisms in a way that suggests the genealogical relationships branching sequences and therefore presents an epitome of evolutionary history.
It also may attempt to show where there are important differences among the various groups. These goals often conflict. In a purely genealogical system, each group must correspond to a single lineage clade composed of the common ancestor and all of its descendants. A group that does not meet both of these requirements is called a grade and may be used as an informal group.
Groups that do not contain the common ancestor, and therefore had two separate origins, are said to be polyphyletic. Such polyphyletic grades, which would put whales together with fish or birds together with bats, have generally been abandoned as soon as they were recognized. Another kind of grade, which does not include all the descendants of the common ancestor, is said to be paraphyletic and is retained in more conservative systems. Within the vertebrates the class Aves is a clade, but the class Reptilia is a grade, for the birds are modified dinosaurs.
Some systems do not recognize Reptilia as a formal group. Likewise, birds, mammals, reptiles, and amphibians are all modified fish, and the old class of fishes Pisces is now rarely used. Therefore there is no formal group called Invertebrata. Many differences among systems are quite subjective. This is often the case when a group may be ranked either as a class or as a subphylum. The organizational limits of some groups are also largely a matter of opinion. Some authors have placed the phylum Hemichordata within the Chordata, expressing the close genealogical relationship.