Introduction:

Chordates belong to animal phylum Chordata and comprise vertebrates, together with many closely related invertebrates, urochordates and cephalochordates. Until recently, the invertebrate group, Hemichordata was put under phylum Chordata but is now considered as the separate phylum. Three chordate subphyla that is Urochordata, Cephalochordata and Vertebrata are so grouped on account of containing certain main characteristics i.e.: a) notochord (dorsal fairly rigid rod of vacuolated cells) or vertebral column b) hollow dorsal nerve cord or spinal cord c) pharyngeal slits d) post-anal tail.

Characteristics of Chordates:

• Primary characteristics of chordates Notochord: This is firm but elastic rod (composed of tightly packed vacuolated cells held in position by firm sheath). Notochord lies along inside of dorsal side of body and gives structural support. It can be located in near original form in few invertebrate chordates.
• Dorsal nerve cord: This is a fluid-filled tube of nerve tissue which runs length of animal, dorsal to notochord. It is present in chordates all over embryonic and adult life. In fish and other vertebrates, nerve cord is symbolized by spinal cord that is major communications line of nervous system.
• Pharyngeal gill slits: Pharynx is part of throat instantly behind mouth down toward stomach. Pharyngeal gill slits are pairs of openings through pharynx. Slits serve as water exit holes by which water, drawn through pharynx, is passed out without it continuing down in rest of gastrointestinal tract.
• Muscular post-anal tail: It is that part of animal which extends backward behind anus. Notochord, nerve cord, and myotomes (muscles which are supplied by nerve of spine) extend to tail. Tail is found at some time during the chordate's development and may be prominent or vestigial.
• Blocks of muscle (metameric musculature): These are muscle blocks on either side of body which enclose notochord and nerve cord.
• Triploblastic coelomates: Body structure is composed of three germ layers (layers of embryonic cells) and well-developed coelom (body cavity).
• Bilateral symmetry: Body of chordate is bilaterally symmetrical i.e. if body is separated in two halves through the central axis; each side is a mirror of other.
• Ventral heart: Heart of chordates is ventrally situated with dorsal and ventral blood vessels and the closed blood system.
• Limbs: Chordates usually have four appendages which are in form of arms, legs, wings or fins.
• Endoskeleton: Chordates have the inner skeleton.
• Digestive system: Chordates have the digestive system of stomach and intestine. Food is taken from mouth that may have tongue and teeth. (They eat plants and animals).
• Nervous system: Chordates contain brain and nervous system. They have most well-developed brains and complex nervous systems of all animal phyla.
• Respiration: Chordates take in oxygen and get rid of carbon dioxide from lungs or gills.
• Reproduction: Chordates reproduce sexually.
• Excretion: Chordates get rid of wastes from kidneys and intestine

General Classification of Chordates:

Phylum Chordata made of four subphyla and every subphylum is composed of classes. Every class in turn is composed of orders. Classify protochordates to level of class only and attempt to classify the vertebrates a step further down to the level of order.

• Phylum Chordata
• Subphylum Hemichordata (halfchordates, currently considered by many authorities as a separate phylum)
• Subphylum Urochodata/Tunicata (tail chordates)

 • Subphylum  Cephalochordata (head chordates)

• Subphylum Vertebrata (backbone chordates).

The hemichordates are considered as half chordates as their chordate characteristics are incomplete or not well-developed, and they don't have post-anal tail. Urochordates are noted for the tail (uro-) while cephalochordates illustrate some degree of head (cephalo-) formation. In vertebrates, notochord has been replaced by the true backbone/vertebral column, composed of either cartilage or bone. Examine the different subgroups in each subphylum.

Subphylum      Hemichordata (half chordates)

Class                 Enteropneusta (acorn worms)

Class                 Planctosphaeroidea (extinct)

Class                 Pterobranchia (Cephalodiscus)

Subphylum      Urochordata (tail chordates)

Class                 Ascidiacea (sea squirts) Class Thaliacea (salps)

Class                 Larvacea (Appendicularia)

Subphylum      Cephalochordata (tail chordate)

Class                 Leptocardii (Leptocardia

Three invertebrate subphyla (Hemichordata, Urochordata, Cephalochordata) given are jointly known as protochordates. They indicate primitive form of chordates.

Other classification of the subphylum Vertebrata and the subgroups under it is also given.

• Class: Cyclostomata :-They contain suctorial and circular mouth without jaws. Paired fins and scales are absent.
• Class Ostracoderms: signifies to armored jawless fishes of Paleozoic. Term doesn't frequently appear in classifications today as it is paraphyletic or polyphyletic, and has no phylogenetic meaning.
• Class Placodermi: is the extinct class of armored prehistoric fish, identified from fossils that lived from Silurian to the end of Devonian Period.
• Class Chondrichthyes: They are marine fishes with cartilaginous endoskeleton. Exoskeleton has placoid scales. Like Pristis (Saw fish), Scoliodon (Shark), Narcine (Electric ray), Trygon (Sting ray).
• Class Osteichthyes: They are marine or freshwater fishes having bony endoskeleton. Exoskeleton contains cycloid or ctenoid scales. Like Sardinella (Sardine), Exocoetus (Flying fish), Anabas (Climbing perch), Labeo (Rohu).
• Class Amphibia: They are vertebrates adapted to land and water. They don't have exoskeleton. Like Rana (Frog), Bufo (Toad), Ichthyophis (Limbless amphibian).
• Class Reptilia : They are creeping or crawling vertebrates possessing dry, cornified skin without skin glands. Exoskeleton contains horny scales or scutes. Like Chameleon (Tree lizard), Hemidactylus( Wall lizard), Chelone (Turtle), Bangarus (Krait), Testudo (Tortoise).
• Class Aves: They are animals having wings and are adapted for flight. They contain feathers as exoskeleton and their jaws are altered into beaks. Like Columba (Pigeon), Pavo (Peacock), Psittacula (Parrot), Aptenodytes (Penguin), Neophron (Vulture), Struthio (Ostrich).
• Class Mammalia: They have breast glands (mammary glands) and most of them are viviparous. They contain hair as exoskeleton. Like Ornithorhynchus (Platypus), Pteropus ( Flying fox), Homo sapiens (Human), Elephas (Elephant), Canis (Dog), Equus (Horse).

1) Hemichordata:  

The subphylum Hemichordata has the following characteristics:

• Body is separated in three sections - proboscis, collar and trunk and bilaterally symmetrical
• Primitive notochord is limited to proboscis only and therefore known as stomochord
• Body contains more than 2 cell layers, tissues and organs
• Straight or U-shaped gut, with the anus
• Nervous system usually diffuse, but variable

The hemichordates are distinguished by the tripartite (three parts) division of body. At anterior end of body is the proboscis (pre-oral lobe); behind this are collar, and finally a trunk. Hemichordates share some of typical chordate characteristics. They contain bronchial openings, or gill slits, which open in pharynx; there is rudimentary structure in collar region known as stomochord, or diverticulum (blind sac) which is composed of cells which resemble those found in notochord but which is not true notochord. There is dorsal nerve cord, additionally to smaller ventral nerve cord.

Characteristics of Class Enteropneusta (Acorn worms):

Enteropneusts included the majority of hemichordates with more than 70 species. They are typical acorn worms and extremely well fit explanation of hemichordates above. They contain the following features:

• Body is divided in 3 sections - a proboscis, a collar and a trunk.
• Numerous bronchial openings, as many as 200 in some species
• Gaseous exchange happens over whole body as well as in pharyngeal slits
• Reproduction is usually sexual involving two opposite sexes and egg fertilization. Though, reproduction occurs as a result of fragmentation of adult body.
• Feeding is either substrate eating or filter feeding.

Enteropneusta (acorn worms) are 2 to 2.5m long; marine in shallow waters, lonely, dwell in mud or vegetation; filter-feeders. They contain well-developed gill slits, and stomochord. They also contain a dorsal strand of nerve cells, thought to be precursor to dorsal hollow nerve cord. Enteropneustes are slow burrowers.

Characteristics of the Class Pterobranchia:

The class is characterized by the given characteristics:

• proboscis is altered in shield.
• collar is altered to generate between 1 and 9 pairs of tentacles or lophophore arms with double row of smaller ciliated tentacles.
• trunk is small and sac-like rather than being long and thin
• asexual reproduction is by budding and is common and frequently gives rise to colonies starting from single individual. Sexual reproduction is through normal method of reproduction as in enteropneusts with external fertilisation.

In Pterobranchia (pterobranchs) there is no trace of the dorsal nerve cord or notochord; have only one pair of gill slits in species of the genus Cephalodiscus. Pterobranchs are the doubtful group of animals, which unlike acorn worms; create colonies in which individuals are interrelated by stems, or stolons. Proboscis is not lengthened, as in acorn worms, but shield- shaped. Collar bears a pair of branched tentacles which collect small food particles from water.

2) Urochordata:

Features of Subphylum Urochordata (Tunicata):

• notochord present only at developmental tadpole stage; absent in adult stage therefore, adult has no endoskeleton
• body completely covered by structure known as tunic composed of secreted protein and cellulose-like material
• body contains more than two cell layers and comprises tissues and organs - triploblastic
• body has no coelomic body cavity
• nervous system made up of anterior ganglion from which individual nerves issue arise
• gill slits are utilized to trap food particles in filter feeding
• haemocyanin as blood pigment (no hemoglobin)
• no excretory organs

Urochordates are medium sized group of marine animals usually referred to as Sea squirts, Tunicates, Salps or Larvaceans. They are all filter feeders using the essentially similar mechanism of pumping water by the perforated pharynx that gathers small particles in layer of mucus. All urochordates contain external covering or 'house' known as tunic that is composed of secreted proteins and polysaccharide much like cellulose. Subphylum is divided in 3 classes: Ascidiacea, Thaliacea and Larvacea.

Characteristics of the Class Ascidiacea:

This class is signified by Sea squirts and they make up bulk of species found within phylum Urochordata. The features of Class Ascidiacea are given below:

• Notochord and post-anal tail is there in larval stage only
• Tadpole-like larvae metamorphose in adults
• Sessile (non-moving or staying in one place) adults
• Marine habitat - the majority species are common coastal animals happening in rock pools and out in deeper water to approx 400- 5,000 m in depth
• Lonely or colonial - colonial species may share common exhalent siphon.
• Translucent or whitish body color.
• Tunic made of mostly of acellular (not made of cells) matrix of tunicin, polysaccharide similar to cellulose
• two openings, inhalant siphon (where water comes in) and exhalent siphon (where water goes out)

Tunicate larvae don't feed and are fundamentally a dispersal form. They soon find the appropriate spot on sea floor and settle in head down, tail up position. They join themselves to sea floor (substrate) through special adhesive glands in front of head and then suffer amazing metamorphosis in which post-anal tail and notochord are lost. Tunicates feed by drawing water in by inhalant siphon. Water passes by pharynx where small particles are trapped before water leaves body by exhalent siphon. Mucous is secreted by special cells and is moved across surface of pharynx by beating of several small cilia, finally it is passed in digestive tract where both particles and it caught up in it are digested.

Features of Class Thaliacea (salps):

Characteristics of the class Thaliacea are given below:

• two-generation life cycle - one generation is lonely and other forms chain-like colonies.
• feed as they swim slowing through warm waters
• filter feeders
• small barrel-shaped animals
• inhalant and exhalent siphons at opposite ends of bodies

The class has approx 70 species. Between two siphons, water passes by several pore or slats of enlarged pharynx that occupies most of body cavity. Tiny particles of plankton are gathered on film of mucus that constantly passes across pharynx. Mucus is secreted by special cells and is kept on moving through beating of many small cilia until it is swept in digestive tract.

Characteristics of Class Larvacea (Apendicularia):

The Larvaceans, at times known as apendicularians, are small animals and are fairly different in form to rest of Urochordata.

Their characteristics are given below:

• planktonic (mass of floating organisms)
• body comprises of basically oval trunk and relatively long thin tail
• tail has notochord which is retained all through animals life, unlike rest of urochordates where it is lost prior to maturity, or even in embryogenesis
• they secrete the gelatinous 'house' which encases trunk or body, but not tail

3) Cephalochordata Subphylum Cephalochordata:

This class comprises several species of lancelets, or amphioxi, tiny, fishlike, filter-feeding animals discovered in shallow water. The lancelet contains the long body, pointed at both ends, with large notochord which extends almost from tip to tip and is there throughout life. It has closed blood circulatory system.

Class: Leptocardii/Leptocardia (small heart):

• No true heart. Heart is represented only by a simple pulsating vessel.
• The blood is colorless
• No brain, renal organs, and limbs.
• Backbone is represented only by a simple, unsegmented notochord

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