AEROBIC AND ANAEROBIC
Some of the spindle fibers reach the chromosomes and attach to protein structures at the centromeres, called kinetochores, while others make contact with microtubules coming from the opposite pole. An infectious disease caused by sporozoans of the genus Plasmodium. The life of birds, fourth edition. Partial Migrant An animal species where some individuals migrate but others don't. Metabolism A mixture of chemical processes that occur within the body of an animal to either release energy breaking down food or to consume it muscle movement. The advantage is that air, high in oxygen content, always moves unidirectional through the lungs. Primary Growth of Roots Concept
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. Others prefer to keep them as a separate phylum because hemichordates lack what are considered important chordate features.
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Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. Aug 30, See Article History. Learn More in these related Britannica articles: Possession of the notochord is what distinguishes members of the most-advanced phylum, Chordata. In the sea squirts Urochordata , the notochord is present in the tail region of the larva but disappears after the animal transforms into the adult.
In amphioxus Cephalochordata , the…. The phylum Chordata is separated into three subgroups or subphyla. The invertebrate subphylum Tunicata consists of the marine tunicates, including the ascidians and salps.
The invertebrate subphylum Cephalochordata includes the fishlike amphioxus or lancelet. Amphioxus is a small marine animal that closely resembles the…. All chordates at some time in their life have a rodlike bar called the notochord running the length of the body.
Lower chordates acorn worms, tunicates, and amphioxus , which lack a vertebral column, illustrate the most primitive features of the chordate nervous system. Phylum Chordata Elastic notochord used as skeleton when present; dorsal hollow nerve cord; postanal tail; pharyngeal slits used originally in feeding and later in gas exchange; all habitats and feeding modes, but rarely parasitic; 50, species.
Subphylum Tunicata Urochordata Secrete a thick tunic or a filtering…. More About Chordate 10 references found in Britannica articles Assorted References annotated classification In animal: Fauna free-swimming behaviour In nekton anatomy and physiology circulatory systems In circulatory system: Chordata endocrine systems In endocrine system: Phylum Chordata enterocoelomate development In enterocoelomate nervous system evolution In nervous system: Others may breathe atmospheric air while remaining submerged, via breathing tubes or trapped air bubbles, though some aquatic insects may remain submerged indefinitely and respire using a plastron.
A very few Arachnids have adopted an aquatic life style including the Diving bell spider. In all cases, oxygen is provided from air trapped by hairs around the animals body. All aquatic reptiles breathe air into lungs. The anatomical structure of the lungs is less complex in reptiles than in mammals , with reptiles lacking the very extensive airway tree structure found in mammalian lungs. Gas exchange in reptiles still occurs in alveoli however, reptiles do not possess a diaphragm.
Thus, breathing occurs via a change in the volume of the body cavity which is controlled by contraction of intercostal muscles in all reptiles except turtles. In turtles, contraction of specific pairs of flank muscles governs inspiration or expiration. See also reptiles for more detailed descriptions of the respiratory system in these animals. Both the lungs and the skin serve as respiratory organs in amphibians.
The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of mucus from specialized cells. While the lungs are of primary importance to breathing control, the skin's unique properties aid rapid gas exchange when amphibians are submerged in oxygen-rich water. The respiratory system of birds differs significantly from that found in mammals, containing unique anatomical features such as air sacs. The lungs of birds also do not have the capacity to inflate as birds lack a diaphragm and a pleural cavity.
Gas exchange in birds occurs between air capillaries and blood capillaries , rather than in alveoli. See Avian respiratory system for a detailed description of these and other features. Many aquatic animals have developed gills for respiration which are specifically adapted to their function. In fish, for example, they have:. In osteichthyes , the gills contain 4 gill arches on each side of the head, two on each side for chondrichthyes or 7 gill baskets on each side of the fish's head in Lampreys.
In fish, the long bony cover for the gill the operculum can be used for pushing water. Click images to enlarge. A cross-section through a trachea TR in the antenna of the rove beetle Aleochara bilineata , illustrating the taenidia TE and a small tracheole Tr branching off.
The trachea is supplying nerves and muscles and is bathed in haemolymph HE. The tracheoles may terminate on the surface of a cell, such as a muscle cell, or they may penetrate inside the cell, either part way or even forming an extensive network inside and also covering the outside of the muscle. The supply is generally greater to flight muscles, especially of the fibrillar type see insect locomotion.
Even individual The Role of Fluid in the Tracheoles When an insect is at rest the ends of the tracheoles are filled with fluid. Textbooks sometimes state that this fluid is needed to dissolve the oxygen.
However, oxygen diffuses faster in air than it does in water, and the fluid is actually a barrier to oxygen diffusion. Thus, when an insect exercises the fluid gets 'sucked' into the muscle cell until oxygen reaches the ends of the tracheoles. This happens, at least in part, as the concentration of solutes build up in the exercising muscle, drawing in the water by osmosis.
The fluid is there at rest because the tissues are bathed in fluid, though it may serve to reduce water-loss and dehydration through the tracheal system. When an insect hatches from its egg, its tracheal system is initially filled with fluid, but this fluid is actively absorbed by the tracheole cells until the system fills with air.
The Transport of Air through the Tracheal System In the basic system described so far, air simply diffuses in through the spiracles and along the tracheae.
Diffusion is rapid over a millimetre or even a centimetre, but is very slow at greater distances. This diffusion-driven system occurs in some small or not very active insects. In large and active insects, such as moths, butterflies, bees and wasps, diffusion alone is insufficient. These latter insects show breathing movements - that is they actively pump air through the tracheal system.
This is why the abdomen pulses in these insects. Sometimes only the tergum of each abdominal segment moves up and down, as in beetles, or both the sternum and tergum, as in flies, or the side-walls pleura may be very flexible and also move in and out, greatly changing the internal volume of the abdomen, a sin moths and butterflies. In this way a rapid stream of air flows through the tracheal system.
Certain spiracles may be used to take air in, others to expel air, e. However, these circuits are not hard and fast and occasionally the direction of flow may be reversed.
Air sacs may facilitate this movement of air through the tracheae. Air sacs can occur in almost any part of the system, and in rigid structures like the head and thorax they may be permanently expanded, acting as reservoirs of air, whilst in the abdomen they may greatly inflate and contract flatten and empty.
The diagrams below illustrate the air sacs in the abdomen of a honey-bee worker. You may have noticed how the abdomen of a honey-bee pulsates as the muscles of the abdomen expand and contract the abdominal segments to fill and deflate the air sacs.
If you have ever chased a bee or wasp, then you may also have noticed that the abdomen pulsates harder and faster with exertion!
Control of Spiracle Opening and Closing Insects lose most of their water through the spiracles. Being small they do not have much water to lose! It is not surprising, therefore, that insects typically only open their spiracles when they need more oxygen. An increase in carbon dioxide, a product of respiration, causes the spiracles to open more. Starvation and reduction in metabolic rate causes them to opening at the end of the trachea, opening at the end of the trachea, rather than the outermost opening of the rather than the outermost opening of the that is the internal opening of the atrium, that is the internal opening of the atrium, atrium a naming convention I personally find unhelpful since in insects lacking atria, the spiracle opens directly to the outside and in diagrams the external openings are generally labeled as 'spiracles' regardless and so I find myself referring to the most external opening, or even the whole structure, as a spiracle, atrium or not.
The external opening of the atrium is often screened by spines, meshes and similar structures may cover the atrial openings in some insects, especially those from dry conditions, though both the spines and the atrium may trap water moisture, they may actually be more important in preventing dust from entering the tracheae.
An internal spiracular valve often occurs between the spiracle and atrium, which can open or close the spiracle.
Temperature is an important factor. At low temperatures, when metabolism may be low, the spiracles are largely closed but open occasionally. At higher temperatures they may open and close periodically, and at still higher temperatures they may open continuously.
Respiration in Aquatic Insects Insects have to be able to obtain oxygen if they are to survive when submerged under water. Some insects have what is called a plastron mechanism - the hairs on their bodies are specially modified to trap a film or bubble of air when they dive these hairs are designed to resist collapse under high pressures.
In Dytiscus Great Diving Beetle and Notonecta Water Boatman air is trapped beneath the elytra wing covers and this air communicates directly with the insect's spiracles openings to its airways or tracheae. These plastrons and sub-elytral air-spaces can function as what are called 'physical gills' - trapped pockets of air that can actually absorb more oxygen from the surrounding water.
This maintains the air bubble for longer and in experiments in which the air was replaced by pure oxygen, the insect actually had to resurface for fresh oxygen sooner than when air was used, since with oxygen the partial pressure of oxygen in the bubble is always greater than in the surrounding water and although the insect takes more oxygen with it to begin with, it is unable to extract any from the surrounding water. In Notonecta the Water Boatman the hindlegs are used to drive water currents over the physical gills to irrigate them with fresh oxygenated water.
Eventually all the nitrogen in the air space dissolves and then the insect must resurface to replenish its supply of air. In Dytiscus and Notonecta the trapped air volume can be regulated when under water, acting to regulate buoyancy.
Structures that have a higher affinity for air than for water, like plastron hairs, are called hydrofuge structures. Hydrofuge hairs also exist on the siphons of mosquito larvae. Some aquatic insect larvae, like mosquitoes, breath through a siphon - a tube with a spiracle at its tip that leads straight into the tracheal airways inside the insect.
The spiracles are surrounded by hydrofuge hairs which repel water and prevent it from entering the system and drowning the insect.
The water trough in the neighbouring meadow has dozens of these larvae that wriggle for cover whenever one's shadow passes over them. The hydrofuge hairs cannot repel oil, which is used to control mosquitoes by applying a film of oil to the water's surface; when the larvae try to breathe the oil enters through the spiracle and they drown.
Eristalis , a type of hoverfly, has aquatic larvae that are called rat-tailed maggots because of the long tail-like extensible breathing siphon. Some aquatic insects have a closed tracheal system which does not open to the outside air via spiracles. These include the dragonfly nymphs which have 6 double rows of lamellae lining the rectum and forming the branchial basket.
Water is pumped over these tracheal gills , mostly in and out through the anus, and oxygen diffuses across to the trachea which fill the gills. Some aquatic insects have spiracular gills , like Simulium black fly pupae. Bloodworms are red worm-like aquatic insect larvae that are often found at the bottom of ponds.
These are the larvae of midges, like Chironomus , and certain flies belonging to the true-flies or Diptera. They are red because they contain a form of haemoglobin which contains 2 haem groups per molecule instead of 4 as are found in vertebrate haemoglobin in their haemolymph 'blood plasma'. This haemoglobin is used to store oxygen, enough for two days in low oxygen conditions as might occur in warm stagnant water. When oxygen is adequate they return to tracheal respiration and replenish their oxygen reserves.