Chordate

Chordate Definition - Every member of the phylum Chordata, which comprises the most highly developed species, the vertebrates (subphylum Vertebrata), as well as two other Subphylum of Chordata is Tunicates (subphylum Tunicata) and cephalochordates (subphylum Cephalochordata). Hemichordata is a phylum that includes chordates in some classifications.

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A chordate has a rigid, dorsal supporting rod at some point during its life cycle, as the name suggests (the notochord). A tail that stretches behind and above the anus, a hollow nerve cord above (or dorsal to) the gut, gill slits that open from the pharynx to the outside, and an endostyle (a mucus-secreting structure) or its derivative between the gill slits are all characteristics of chordates. (A distinguishing feature can appear only in the developing embryo and then vanish as the embryo matures into an adult.) Hemichordata, a closely related phylum, has a body plan that is very similar.

Chordata Meaning - The vertebrates, as well as sea squirts and lancelets, form a broad phylum of animals. They can be identified by the presence of a notochord at some point during growth.

Scientific Classification 

• Kingdom - Animalia  

• Subkingdom - Eumetazoa

• Clade - Parahoxozoa

• Clade - Bilateria

• Clade - Nephrozoa 

• Superphylum - Deuterostomia 

• Phylum - Chordata

General Features 

Tunicates are small animals that range in size from one to five centimetres (0.4 to 2.0 inches) in length, with a minimum length of one millimetre (0.04 inch) and a maximum length of slightly more than 20 centimetres; colonies can reach a length of 18 metres (59 feet). Cephalochordates are one to three centimetres in length. Vertebrates vary in scale from small fish to whales, which are among the world's largest mammals.

Tunicates are benthic (bottom dwellers) or pelagic (open water dwellers) marine animals that form colonies through asexual reproduction. They consume water through their mouths, filtering it through the gill slits. Cephalochordates have a similar feeding apparatus. They have strong musculature and can swim quickly by undulating their bodies. Cephalochordates spend most of their lives partly submerged in sand and gravel in the ocean.

Vertebrates, including tunicates and cephalochordates, have traces of a feeding apparatus. The gill slits, on the other hand, stopped functioning as feeding mechanisms and then as breathing devices as the vertebrate system evolved. A pair of gill arches have been adapted to form jaws, except in certain early branches of the vertebrate lineage (e.g., agnathans). The creation of fins, which were later converted into limbs, changed the fishlike habitus that started with cephalochordates. The introduction of vertebrates into fresh water and then onto land resulted in a change in breathing mechanisms, from gills to lungs. Other changes, such as an egg that could hatch on land, freed the vertebrates from their dependence on water. Amphibians, reptiles, birds, and mammals arose from the diversification of structure and function enabled by the invention of the locomotory apparatus and other innovations.

Reproduction and Life Cycle 

The life cycle of a chordate starts with fertilisation (the union of sperm and egg). Fertilization takes place externally, in the water, in its simplest form. In tunicates and certain vertebrates, asexual reproduction occurs (females of some fish and lizards can reproduce without fertilization). Hermaphroditism (possessing both male and female reproductive organs) is found in tunicates and some fishes, but otherwise, the sexes are separate. Larvae (very young forms that differ considerably from the juveniles and adults), when they do occur, differ in structure from the larvae of non-chordates. Internal fertilization, viviparity (giving birth to young that have undergone embryological development), and parental care are common in tunicates and vertebrates.

Ecology and Habitats

Chordates can be found in a variety of environments. Tunicate larvae either look for a place to attach and metamorphose into adults, or they grow into adults that float in open water. Cephalochordates grow up in open water, but as adults, they are covered partially or completely in sand and gravel. They are filter feeders with basic behaviour in either case. Vertebrates have a much more complex ecology and habits, which is in line with their more aggressive mode of food acquisition.

Locomotion 

Chordates can travel about by using muscle movements at any point in their lives. This is achieved in tunicate larvae by a tail; in cephalochordates by body undulations; and in vertebrates by general body movements (as in eels and snakes) and the motion of fins and limbs, which are transformed into wings in birds and some mammals.

Associations 

Chordates are involved in a wide range of symbiotic relationships and are particularly notable as parasite hosts. Because of their complex nervous systems, vertebrates have especially well-developed family groups and societal relationships in both a wide and narrow sense. This phenomenon can be observed in fish schools, bird flocks, and mammalian herds, as well as primate associations that may indicate the origins of human civilization.

Form and Function 

Chordates have several distinguishing characteristics, indicating that they have undergone substantial evolution from their basic origins. The mouth that forms separately from the anus, as it does in the phyla Hemichordata, Echinodermata, and Chaetognatha, is a feature shared with some invertebrate phyla in the early stages of chordate formation. Similarly, the coelom, or secondary body cavity around the viscera, develops as outpouchings of the gut in these phyla. Some more distantly related phyla, such as Annelida, Arthropoda, and Mollusca, also have a coelom, but the main organs of the body are organised differently in these phyla. The main nerve cord is single in chordates and lies above the alimentary tract, while it is paired in other phyla and lies below the gut. Both cephalochordates and vertebrates, as well as of annelids and their ancestors, have segmented bodies; however, segmentation in the two classes likely evolved independently. The gill slits and some other characteristics shared by hemichordates and chordates evolved before the chordates became their own group. Hemichordates lack a tail above the gut and an endostyle that secretes mucus between the gill slits.

The adult lancelet and tunicate tadpole larvae indicate that an ancestral chordate had a distinct front and back end, an anterior mouth, a posterior tail over an anus, unpaired fins, and gill slits that opened directly to the outside. An associated, sessile adult tunicate with an atrium that surrounds the gills metamorphoses from a free-swimming tunicate larva. Lancelet atriums are thought to have formed independently.

Internal Features 

Skeleton and Support 

The chordate notochord is a rigid rod with a fibrous sheath and a turgid heart. When locomotory waves are formed by muscular contraction, it prevents the animal from shortening. Fluid in the body cavities supports the chordate body. The tunic provides additional support in tunicates. The gills and other body parts of tunicates and cephalochordates are supported by cartilaginous material. The skeletons of immature vertebrates are still cartilage, which becomes bonier as they get older. Sharks and other vertebrates' cartilaginous skeletons are believed to have formed from more heavily mineralized skeletons.

Tissues and Muscles 

Both cephalochordates and vertebrates have well-developed and segmentally organised locomotion muscles. Tunicates have a simpler tail musculature with no obvious signs of segmentation. Both chordates have a small amount of musculature in their bodies. The muscles that control jaws, arms, and other body parts have developed alongside them in vertebrates.

Nervous System and Sense Organs 

Chordates have an expanded anterior end of the main nerve cord that resembles a brain, but only vertebrates have a fully formed brain. Tunicate larvae have light-sensitive visual organs as well as sense organs that respond to gravity. Lancelets' nerve cord contains pigment spots and light receptors that sense abrupt shifts in light intensity. Vertebrates have more intricate and complex eyes and other sense organs.

The existence of a nervous system of segmentally replicated nerves originating from the dorsal hollow nerve cord in cephalochordates and vertebrates suggests shared ancestry. Segmentally replicated nerves do not exist in the tunicate nervous system. Both vertebrates' brains are massively expanded and divided into functionally distinct regions.

Digestion and Nutrition 

Tunicates and cephalochordates are also filter feeders with small food particles suspended in water. The gill slits' beating cilia (hairlike cellular extensions) attract water into the mouth and into the pharynx, where a layer of mucus secreted by the endostyle (a glandular organ located underneath the two rows of gill slits) filters suspended food particles from the water. The food-rich layer of mucus is moved upward over the gill slits by cilia lining the pharynx, then rolled up and transported to the gut's posterior portion. The atrium's water current enters and exits through the atrial opening.

The vertebrates in the “ammocoetes” larva stage of the primitive jawless fish known as the lamprey have a similar structure. The difference is that the food is made up of larger particles that have settled to the bottom (detritus), and the pharyngeal musculature pumps water and food particles through the gill slits instead of cilia driving the feeding current. The earliest fishes most likely ate detritus, and their sucking behaviour is still seen in modern fishes (lampreys and hagfishes). With the evolution of jaws, vertebrates gained the ability to grab and seize larger food items. The primitive chordate's lower digestive tract is a simple duct with a sac-like stomach. Only examples of specialised areas and glandlike structures found in vertebrates, such as the liver and pancreas, exist.

Excretion 

The kidneys are primarily responsible for waste excretion and controlling the chemical composition of the internal atmosphere, though other body parts, such as the gills, can also play a role. With the exception of hagfishes, tunicates and cephalochordates have body fluids with a salt content similar to that of seawater, but vertebrates, including aquatic animals, have low salt content body fluids. One theory is that the vertebrates formed in freshwater, but it seems more likely that hagfishes split off while still in the sea, and the freshwater form evolved later.

Respiration 

Tunicates and cephalochordates have a primitive chordate gill, which is used for both respiration and feeding. Since the current is now formed by muscles rather than cilia, the vertebrate gill may still play a role in feeding. In different lineages, the gills were less in size, and they were reinforced by supporting components, some of which developed into jaws. Lungs, which were already present in fish, evolved into the primary respiratory organ of terrestrial vertebrates.

Circulatory System 

In chordates, the circulatory system follows a specific pattern. Blood is driven by a distinct heart in tunicates and vertebrates, and by blood vessel contraction in the cephalochordates. The ventral aorta is a vessel that transports unoxygenated blood forward. It then travels via a series of branchial arteries in the gills, where gas exchange occurs, and oxygenated blood flows to the body, with most of it returning to its source through a dorsal aorta. The blood of vertebrates circulates through the tissues through capillaries, which are tiny blood vessels. Capillaries are absent in tunicates and cephalochordates, so blood flows through gaps in the tissues instead.

Hormones 

Endocrine glands (those of internal secretion) in vertebrates produce hormones that regulate a variety of physiological activities. Organs that correspond in anatomical location to the pituitary gland of vertebrates have been described in tunicates and cephalochordates, but it is unknown which hormones if any, they secrete. The thyroid gland in vertebrates develops thyroxine, an iodine-containing hormone that aids in metabolism regulation. The thyroid is a modified endostyle, as shown by the fact that the thyroid of larval lampreys also secretes mucus for feeding. Lancelet endostyles absorb iodine and produce thyroxine, but the thyroxine produced does not act as a hormone in the lancelets.

Features of Defence and Aggression 

Tunicates rely on their heavy tunic's passive defence to a large extent. Lancelets travel quickly through the substrate, and their highly developed locomotory apparatus evolved primarily to help them avoid predators. Vertebrates have stopped consuming detritus carried by water flows. They've started eating bigger foods and are more good at spotting down, pursuing, and subduing what they consume.

Evolution and Paleontology

Many scientists believe chordates evolved before 590 million years ago, implying that they predate the fossil record. Since these early members were soft-bodied, they left a smattering of fossils. Pikaia gracilens, a primitive cephalochordate that lived 505 million years ago, is the oldest known fossil chordate. There is debate about whether older species, such as Yunnanozoon lividum and Haikouella (both of which are 530 million years old and have some chordate characteristics), should be classified as chordates. About 400 million years ago, a large number of vertebrate fossils were discovered.

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The deuterostomes (bilaterally symmetrical animals with indeterminate cleavage and whose mouth does not emerge from the blastopore) include the phyla Hemichordata, Echinodermata, and Chaetognatha, according to embryological evidence. Hemichordates are the chordates' closest relatives since these species have gill slits and other characteristics not present in other animal phyla. On the basis of resemblances between the larvae of certain hemichordate groups and echinoderms, a slightly more distant relationship to the echinoderms is hypothesised. On the basis of characteristics such as what appear to be gill slits, the derivation of chordates from some fossil echinoderms has been argued. There have been proposals to derive them from other phyla (e.g., Annelida, Nemertea, and Arthropoda), but such ideas have few contemporary supporters.

It's been widely discussed whether the first ancestral chordate was more of a tunicate or a cephalochordate. According to the traditional theory, the ancestor was similar to a cephalochordate, with one lineage being attached to hard surfaces and evolving into tunicates, while the other remained unattached and developed into vertebrates. Another hypothesis is that the ancestor was similar to a tunicate, and the other two subphyla developed as a result of tadpole larval alteration. The classical theory is preferred because it offers the most satisfactory explanation for the similarities between chordates and hemichordates of the Enteropneusta subphylum. The tunicates branched off from the chordates before the common ancestor of cephalochordates and vertebrates emerged since the latter is similar in certain aspects of neuroanatomy and biochemistry.

Development 

This document describes the major chordate groups. Modern systematic biology tries to organise groups of species in such a way that their genealogical relationships (branching sequences) can be inferred, and thus provides an epitome of evolutionary history. It may also attempt to demonstrate where significant differences exist among the different classes. These objectives often clash. Each category must correspond to a single lineage (clade) consisting of the common ancestor and all of its descendants in a strictly genealogical scheme. A grade is a category that does not follow any of these criteria and may be used as an informal group.

Polyphyletic groups are those that do not have a common ancestor and therefore have two distinct roots. Polyphyletic grades that lumped whales and fish together, or birds and bats together, were usually discarded as soon as they were discovered. Paraphyletic is a form of grade that does not include all of the descendants of a common ancestor and is used in more conservative systems. The class Aves is a clade within the vertebrates, but the class Reptilia is a category since birds are modified dinosaurs. Reptilia is not recognised as a formal entity in some systems. Birds, rodents, reptiles, and amphibians are all transformed fish, and the old fish classification (Pisces) is no longer used. While vertebrates are a single clade, the term "invertebrate" refers to all animals other than vertebrates. As a result, there is no formal Invertebrata category.   

Many of the distinctions between systems are highly subjective. When a group is classified as a class or a subphylum, this is often the case. Some groups' organisational boundaries are often largely subjective. The phylum Hemichordata has been placed within the Chordata by some scholars, indicating a similar genealogical relationship. Others tend to retain hemichordates as a separate phylum because they lack essential chordate characteristics.

Phylum Chordata facts

• Above the notochord, which is gelatin-like and encased in a tough membrane, is a tubular nerve cord (such as the spinal cord).

• Gill slits lead into the throat or pharynx among all of them.

• Even if they don't have blood cells, they all have blood vessels that contain blood.

• All have a tail that stretches past the backbone and anus and contains no internal organs.

Conclusion

Chordates are species that have a structure called the notochord at some point during their growth into a mature organism. In this article, we will come across what are chordates? definition, meaning, characteristics and also about their external and internal features in brief.  Cephalochordate, Vertebrata, and Urochordata are the three subphyla of the Chordata phylum. Chordates have a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail, among other features.