Phylum Chordata:

Meaning of Phylum Chordata:

The Phylum Chordata encompasses a vast group of diverse animals ranging from ascidians to man. A few characteristics like Notochord, Dorsal hollow tubular nerve cord and Pharyngeal gill-slits unite these diverse animals under a common phylum.

They differ from the non-chordates significantly by the position of nerve cord. The non-chordates possess a ventral solid nerve cord below the alimentary canal, while a dorsal hollow nerve cord above the gut is the diagnostic character of the chordates. The study of Phylum Chordata is of special interest to us, because we, the human beings, are also included within this group.

2. Important Features of Phylum Chordata:

The members of the Phylum Chordata possess many features in common.

The most important features are:

(a) The notochord,

(b) The dorsal tubular nerve cord,

(d) A post-anal tail.

These four characteristics are unique to the phylum. Existence of such common structures is interpreted as a result of inheritance from a common ancestry. Besides these four basic structures, there are a few other characteristics which have less diagnostic value.

(d) A post-anal tail.

Notochord or Chorda Dorsalis:

The notochord is the prime identifying structure of the chordates and the group derives its name from it. It is a rod-like elastic structure situated just above the alimentary canal and immediately beneath the dorsal tubular nerve cord. The notochord is composed of special type of vacuolated notochordal cells and remains covered over by one or two connective tissue coverings, called notochordal sheath.

The notochord (Stomochord) in the hemichordates originates from the embryonic endoderm but in other chordates, it is a mesodermal derivative. The stomochord is not supportive and possesses a cavity that opens into the pharynx. The notochord may persist even in the adult primitive chordates, but in vertebrates it is either partially or wholly replaced by the vertebral column.

Dorsal Tubular Nerve Cord:

In chordates, a single dorsal tubular nerve cord extends along the anteroposterior axis of the body. It is located just above the notochord. This nerve cord originates from the embryonic dorsal ectoderm. It develops as an oval dorsal plate called neural or medullary plate. The neural plate is formed as a result of assemblage of (neural) cells.

The neural plate subsequently transforms into a neural or medullary groove possibly due to the lateral pressure exerted by the surrounding non-neural tissue. The dorsal tips of the neural groove become fused with each other and a neural tube is formed.

The cavity within the neural tube is called neurocoel. In the invertebrate chordates the dorsal tubular nerve cord remains almost unchanged, but in the vertebrates the anterior region of the nerve cord becomes specialised into the brain and the remaining part is modified into spinal cord. These two parts remain connected by a short bridge called isthmus.

The development of dorsal nerve cord is as follows:

Pharyngeal Gill-slits:

The gill-slits have many alternative names, such as pharyngeal or branchial clefts, visceral clefts, visceral or branchial pouches. In the majority of the primary aquatic chordates, the pharyngeal wall is perforated by gill-slits which are meant for the exit of water current from the pharyngeal cavity to the outside.

In the majority of such forms the respiratory organs (gills) are lodged in the gill-chamber. Exchange of gases takes place during the transit of the water current that passes through the mouth and goes out through the gill-slits.

The gill-slits, as such, are persistent in invertebrate chordates, fishes, amphibian larvae and in the neotenic adult amphibians. But the transition from the aquatic environment to the land-life has changed the nature of the gill-slits.

The gill-slits lose their importance due to the acquisition of pulmonary respiration. The respiratory function of the gill-slits is taken up by the lungs, the respiratory organs of the land chordates. The initial stages of formation of gill-slits are clearly seen in the embryos of the land chordates. In the adults, these structures become transformed into other organs, specially the endocrine systems.

The gill-slits are developed as a result of subsequent fusion of the in-pouchings of the body ectoderm with the corresponding out-pouchings of the pharyngeal endoderm.

Initially both the in-pouchings and the out- pouchings meet with each other and then the regions of union breakthrough to form continuous passages between the pharyngeal cavity and the exterior. The number of the gill-slits varies considerably in different chordates. Cephalodiscus bears only one pair of gill-slits whereas Branchiostoma possesses as many as two hundred.

Remarks:

According to Kardong (2002) — “The term gill-slits is often used in place of ‘pharyngeal slits’. A ‘gill’ proper is a structure that is composed of tiny plates and harbour capillaries, used for aquatic respiration. In vertebrates, the gills are situated adjacent to the pharyngeal slits. The slits do not play any significant role in respiration, only remain as openings. In many primitive chordates, the openings only serve primarily in feeding but in embryos they play no respiratory role, hence gill-slits is a misleading term.”

Post anal Tail:

Chordates possess a post anal muscular tail containing extensions of the notochord and nerve cord. Though it is a characteristic feature of the vertebrates but it may or may not present in adult stage.

In protochordates, the tail is absent in adult ascidians and hemichordates, whereas in amphioxus, it is present in adult stage. In primitive chordates the tail plays as a locomotors organ but in higher chordates or vertebrates the tail is employed in various functions such as swimming, balancing, prehensile and food capturing.

3. Chordate Features Shared by the Non-Chordates:

Besides the four unique features of the chordates, there are many characteristics which are also present in many higher invertebrate chordates. The significance of the structural similarities is very difficult to interpret from the phylogenetic point of view.

However, it may be suggested that the chordates as a group evolved from some higher groups of non-chordates and, hence, the structural resemblances are due to remote common ancestry.

Bilateral Symmetry:

Both the chordates and most of the non-chordates like annelids, arthropods, etc. exhibit distinct bilateral symmetry.

Axiate Organization:

All the chordates have a distinct polar axis. The anterior end is marked by the presence of head and the posterior end is characterised in most cases by the tail. The axis extending from the head to the tail end is regarded as the anteroposterior axis.

The anteroposterior axis of the chordates corresponds to that of most of the higher non-chordates. The axiate organization is not strictly homologous, because many fundamental differences exist between the two groups.

Triploblastic Condition:

All animals above the rank of cindarian coelenterates have a third germ layer besides ectoderm and endoderm. This third layer is known as mesoderm.

Although the embryonic formation of the mesoderm is different in non-chordates, its formation is similar in chordates, echinoderms, brachiopods, chaetognaths and in some other enterocoelous forms. The triploblastic condition has added more weight to the phylogenetic relationship of the chordates with the non-chordates.

Metamerism:

Segmental organisation is characteristic of most of the non-chordates and the chordates. In annelids and arthropods, segmentation is well-marked both internally as well as externally but in chordates the external segmentation is not seen. The segmental arrangement of the body wall musculature is prominent in chordates.

Coelom:

The eucoelom or true coelom is the secondary body cavity of triploblastic animals, situated between the gut and body wall. The space of body cavity is lined by coelomic epithelium and contains coelomic body fluid.

The mode of origin of coelom is different among the different groups of invertebrates and chordates. In annelids, arthropods and molluscs the coelom formation is of schizocoelic type, because the coelom develops by the splitting of the embryonic mesoderm layer.

In echinoderms, hemichordates and in other chordates the coelom formation is of enterocoelic type, or the coelom is called enterocoel, because the coelom develops from the embryonic archenteron or enteron. Here mesoderm arises in the embryo as paired lateral pouches growing out from the endoderm.

These pouches gradually lose continuity with the endoderm and grow downwards and inwards until they meet and fuse. The inner splanchnic part remains against the wall of developing gut and outer somatic part of the mesoderm becomes applied against the developing body wall.

Embryonic Development:

Protostome and deuterostome are the two groups of animals which differ in the embryonic origin of the mouth. Among protostomes, the mouth is formed from the blastopore, hence protostome means ‘first mouth’. Among the deuterostomes, the mouth does not form from blastopore. Instead it may give rise to anus.

In this group the mouth is the second opening, hence called deuterostome. The differences on the basis of embryological development have strongly supported by analysis of phosphate-containing storage molecules that are found in muscles and are used in the synthesis of ATP.

Protostomes (e.g., Annelids, arthropods and molluscs) contain arginine phosphate and deuterostomes (e.g., echinoderms and chordates) contain creatine phosphate.

Although the chordates share many common features with the non-chordates, the fundamental organisation is different. The differences are shown in Table 4 Chordata.

Nature of Sexes:

Majority of chordates are dioecious (the sexes are separate) but in a few hemichordates and in majority of urochordates the hermaphroditism predominates.

4. Ancestry of Phylum Chordata:

The chordates include organisms having a notochord, a dorsal hollow nerve cord, pharyngeal slits or pouches and a few features like bilateral symmetry, axial organisation, triploblastic condition, segmentation, etc., that are common with the non-chordates. The question of the origin of the chordates still remains unanswered and considerable controversy exists on this issue.

The geological records established beyond doubt that the chordates originated prior to Cambrian period because the relics of some lower chordate forms have been discovered in Cambrian strata. There are various theories regarding the origin of the chordates from the non-chordate groups. Most of the theories suffer from serious defects.

Of all the theories regarding the ancestry of chordates from some non-chordates, Garstang’s suggestion that the chordates have evolved from some free-swimming echinoderm larvae (possibly auricularian larvae) by means of paedomorphosis has been accepted by many workers.

The role of paedogenesis (reproduction in pre-adult stage) in evolutionary dynamics is emphasised by many workers on this line. But in recent years the ancestry of the chordates from the echinoderm source is not accepted.

Recent workers regard the differences between the vertebrates and the non-chordates (invertebrates) to be artificial in nature. Inclusion of the echinoderms, pogonophores and chordates under deuterostomia (animals where the anus develops from the blastopore and the mouth is formed anew) is accepted nowadays. The protochordates (urochordates and cephalochordates) are the members of the Phylum Chordata.

The protochordates provide connecting link between the vertebrates with other deuterostomes. The deutorostomes are highly specialised groups and it will be improper to regard them in the direct line of vertebrate descent.

The phylogenetic status of the hemichordates is a subject of great controversy. But the chordate nature of the urochordates and the cephalochordates is well-established though their relationships with the vertebrates and with each other are difficult to ascertain.

Barrington (1965) suggested that the deuterostomes have evolved from sessile/semi-sessile ancestors having bilaterally symmetrical and tripartite body and coelom. The echinoderms have departed a long way from the ancestors, while the hemichordates remained closer.

The hemichordates have developed pharyngotremy (i.e., existence of openings in the pharyngeal wall) which is associated with its ciliary mode of feeding. In course of time a group with internal food collection mechanism by elaborate and complicated pharynx gave rise to the urochordates, cephalochordates and vertebrates.

5. Outline Classification of Phylum Chordata:

Like many other phyla, the taxonomical subdivisions of the Phylum Chordata are a problematic issue. Diverse opinions exist on this particular aspect. The systematic status of hemichordates has long been a debated issue. In recent years the hemichordates have been separated from the Phylum Chordata and given the rank of a separate phylum.

In this present text the Phylum Chordata is divided into:

Subphylum I. Cephalochordata

Subphylum II. Urochordata

Subphylum III Conodontophorida

Subphylum IV. Vertebrata (Craniata)

The classification of vertebrates is a very difficult task. Despite remarkable advancement of knowledge on this topic, the classification of vertebrates is open to refinement. The term Vertebrata was introduced by Lamarck into the Science of Zoology. But the classification of the Vertebrata into different classes remained still a problem.

Aristotle and Linnaeus established four classes [Pisces, Amphibia (including the reptiles also), Aves and Mammalia] under the Vertebrata. The class Amphibia included the naked Amphibia and the scaly animals (Reptilia).

Pisces and naked amphibia have many features (e.g. branchial respiration, lack of amnion and allantois, persistent notochord, etc.) in common. Similarly the reptiles and birds share many common characters. Huxley adopted three principal groups of Vertebrata.

They are:

I. Ichthyopsida (Pisces and Amphibia);

II. Sauropsida (Reptilia and Aves) and

III. Mammalia.

The Class Pisces has been divided into five subclasses by many authors like Sedgwick (1905). The subclasses are: Marsipobranchii, Elasmobranchii, Ganoidei, Dipnoi and Teleostei.

Chordata is divided into two major groups — protochordates and vertebrates or craniates. Hemichordates, urochordates and cephalochordates are collectively recognised as protochordates. The members of this group do not possess vertebral column. The protochordates often referred to as invertebrate chordates or primitive chordates which are considered as transitional group in between invertebrates and vertebrates.

At present some authors are of opinion to remove Hemichordata from Protochordata or Chordata and treat Hemichordata as a separate Phylum. Vertebrata is recognised for the possession of vertebral column. The differences between protochordates and vertebrates are shown in Table 5.

Vertebrata is divided into two super classes Agnatha and Gnathostomata. Agnatha means mouth without jaws and Gnathostomata means jawed mouth vertebrates. Agnathans are petromyzontids (lampreys) and myxinoids (hagfishes), and gnathostomes are cartilaginous and bony fishes, amphibians, reptiles, birds and mammals.

The terms Tetrapoda (tetrapods) or quadrupeds are often used within gnathostomes. Tetrapoda means four-footed vertebrates which include amphibians, reptiles, birds and mammals but proper meaning for four-footed should be quadrupeds.

From embryological point of view, gnathostomes are divided into Anamniota and Amniota. Amniota is a group of vertebrates whose embryos possess a delicate, transparent membrane, the amnion that remains around the developing embryo.

The sac-like amnion contains a fluid, the amniotic fluid, which protects the developing embryos from extreme cold, desiccation and also from external injury. Vertebrates are without any amnion, called anamniota (fishes and amphibians), and amniotes are reptiles, birds and mammals.

The term “Pisces” is often used for cartilaginous and bony fishes. Pisces are aquatic gnathostomes having gills in adult state, and median fins are supported by a special skeleton. These terms are used by various taxonomists to classify the vertebrates.

Inclusion of the subphyla Urochordata, Cephalochordata and Vertebrata under the Phylum Chordata is of special interest to us from the point of view of evolution. The association of the first two subphyla with the vertebrates suggests the possibility of intergradation between these groups and the sequences for evolutionary transition from one to the other.

This biological association dates back to Lamarck (1809, 1815) and Cuvier (1816) who sponsored the scheme for the first time and recognised the uniqueness of the vertebrates possessing the vertebral column and the invertebrates who lack this feature.

Costa (1938) and Yarrel (1835) recognised Branchiostoma (Amphioxus) lanceolatum as a ‘low vertebrate’ and Kowalevski (1869, 1871) demonstrated the existence of basic similarities between vertebrates and tunicate larvae.

Balfour (1880) established the phylum Chordata which included the vertebrates, Amphioxus and tunicates. Bateson (1885) added the hemichordates to the Phylum Chordata. Fowler (1892) suggested the inclusion of Rhabdopleura in the Subphylum Hemichordata and Spengel (1932) added Planctosphaera to it.

The current opinion is in favour of removal of the Hemichordata from the Phylum Chordata leaving the Urochordata, Cephalochordata, Conodontophorida and Vertebrata as the Subphyla of the Phylum Chordata. The Hemichordata is now given the status of a separate phylum.

However, it is still a common practice in many text-books to designate the hemichordates, urochordates and the cephalochordates as the protochordates/lower chordates/invertebrate chordates. The naming of Hemichordata as the Invertebrate chordates appears to be most convincing.

In the present text, the Hemichordata is described as a separate phylum, recognised as a transitional group between the non-chordates and protochordates (urochordates and cephalochordates). The Phylum Chordata unites the diverse forms having a notochord, hollow dorsal nerve cord and pharyngeal gill-slits.

Patterns of Classification:

Nowadays, more or less two types of classification are used in the classification of animals. The older one is evolutionary classification which depends maximally on fossil record. This classification reconstructs phylogenetic sequences using judgement and makes evolutionary trees based on available data including fossil record.

In this type of classification, the different animals belonging to a single group have a common ancestor. The phylogenetic classification or cladistic pays little attention on fossil record.

It is one method, related to the branching sequences of the phylogenesis, and the phylogeny is reconstructed on the basis of shared derived characters that analyse the primitive and derivative characters. This system of classification performs a good degree of objectivity in the selection of morphological characteristic features that are employed in reconstructing phylogenetic relationship.

A clade is the phylogenetic lineage evolving from a common ancestor. A clade may be monophyletic, polyphyletic or paraphyletic. A monophyletic of all the organisms of a lineage is evolved from a common evolutionary ancestor or ancestral group.

Polyphyletic is one which is characterized by features but is not homologous. Groups of different lineages which have a common ancestor and some members, but not all of its descendants, are paraphyletic. The evolutionary history of a lineage is called phylogeny and when it represents in a graphic scheme is called dendrogram. The cladogram is a simply branching diagram that depicts the relationships of different clades.

Table 6 presents a conventional type of classification of the Phylum Chordata in which Hemichordata is treated as subphylum. Table 7 presents a “Cladogram of Chordata” where species or groups of species have shown their relationships. Table 8 presents the “characteristics of the living chordate classes” and Table 9 presents the “Geological time scale”.