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In this article we will discuss about the Characters and Classification of Chordates.
Characters of Chordates:
All the chordates possess four diagnostic characters either in the embryonic or adult stage.
1. Notochord:
It is a solid un-jointed, stiff but flexible rod-like structure situated on the dorsal side between the dorsal hollow nerve cord and the alimentary canal.
2. Dorsal Hollow Nerve Cord:
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The nerve cord of chordates is always hollow and lies dorsal to the notochord.
3. Pharyngeal Gill Slits:
All the chordates have at some stage of life, a series of paired narrow openings, the gill slits on the lateral sides of the pharynx.
4. Tail:
It is a post-anal part of the body.Which is reduced or absent in many adult chordates.
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Other Characters of Chordates:
These include bilateral symmetry, three germinal layers, segmentation, organ-system level of organisation, cephalization, coelom, endoskeleton, complete digestive tract, special organs for respiration and excretion, closed circulatory system, separate sexes, gonads with gonoducts and without asexual reproduction.
Classifications of Phylum Chordata:
Sub-Phylum 1. Urochordata:
(i) Adults are generally sedentary (fixed to substratum),
(ii) This sub-phylum is also called Tunicata because the adult body is enclosed within a leathery test or tunic formed of a cellulose-like organic substance termed tunicin.
(iii) The notochord is only present in the tail of the larva and disappears in the adult,
(iv) The dorsal tubular nerve cord is found in the larva. It is replaced by a dorsal ganglion in the adult,
(v) The pharynx is perforated by numerous apertures called stigmata formed by the larval gill slits. The stigmata open into an ectoderm lined cavity, the atrium, and
(vi) The larva (tadpole) is motile and undergoes retrogressive metamorphosis, i.e., change from better developed larva to less developed adult.
Examples:
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Herdmania (Sea Squirt), Ascidia, Ciona, Doliolum, Salpa, Botryllus (colonial urochordate), Molgula, Pyrosoma. Pyrosoma is bioluminescent colonial urochordate. Herdmania has valveless heart. The blood of Hardmania is green due to the presence of vanadium in blood.
Ascidia:
It is closely similar to Herdmania. It is a marine, solitary and sedentary, living in temperate seas. The body is attached to the substratum by a foot. Test is thick and tough and branchial siphon and atrial siphon are short. There is a motile tailed tadpole (larva) which undergoes retrogressive metamorphosis to become sessile adult.
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Sub-Phylum 2. Cephalochordata (Gr. cephalos- head + chordata = notochord):
(i) Both the adult and larva are motile,
(ii) The notochord extends up to anterior end of the body hence this subphylum is named,
(iii) The notochord persists throughout life,
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(iv) Pharyngeal gill slits are more numerous and are better developed.
(v) Atrium is also present,
(vi) The tail is present throughout life,
(vii) It shows progressive metamorphosis (change from less developed larva to better developed adult).
Example:
Branchiostoma (= Am- phioxus). Amphioxus has both ends pointed like lance hence it is commonly called lancelet. Amphioxus has numerous eye spots or ocelli which are sensitive to light. Subphyla Urochordata and cephalochordate are often referred to as protochordates or acrania (without cranium — brain box).
Sub-Phylum 3. Vertebrate or Craniata:
Some of the important characters of vertebrata or Craniata Sub-Phylum are listed below:
(1) These are advanced chordates that have cranium (brain box) around brain.
(2) Notochord is only present in the embryonic stage; it is replaced by a cartilaginous or bony vertebral column in the adult forms.
(3) There is very high degree of cephalization (formation of head).
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(4) The epidermis consists of many layers of cells. Epidermis may bear an exoskeleton of scales, feathers or hair.
(5) Three types of muscles, striped, un-striped and cardiac, are present.
(6) Coelom is well developed.
(7) Digestive tract is complete.
(8) The endoskeleton is formed of cartilage or of cartilage and bone.
(9) Heart is ventrally situated with two, three or four chambers. There is present hepatic portal system.
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(10) There is closed circulatory system consisting of blood vascular and lymphatic systems. RBCs are present.
(11) Respiratory organs may be gills, skin, buccopharyngeal cavity and lungs.
(12) A pair of kidneys is present for excretion and osmoregulation.
(13) Nervous system consists of central nervous system (brain and spinal cord), peripheral nervous system (cranial and spinal nerves) and autonomic nervous system (sympathetic and parasympathetic nervous systems).
(14) Sense organs are eyes, ears, tongue, nasal chambers, and skin. In some vertebrates lateral line system is present.
(15) Cranial nerves are 8, 10 or 12 pairs.
(16) Endocrine glands are found in all vertebrates.
(17) Sexes are separating (unisexual) except hag fish, which is bisexual. There is no asexual reproduction.
Subphylum vertebrate is divided into two sections:
Section 1. Agnatha (The lawless Vertebrates):
The mouth does not possess jaws hence named agnatha. Notochord persists throughout life. Vertebral column is represented only by small imperfect neural archs over the notochord. They do not have paired appendages. They have single nostril. Internal ear has one or two semi-circular canals. They are cold blooded. Agnatha has two classes: Ostracodermi and Cyclostomata.
Class 1. Ostracodermi (Extinct):
They are earliest known vertebrates which appeared in Ordovician period. They had well developed dermal scales which led to their names “Ostracoderms”— bony skin. They are also called “armoured fishes”. All are extinct.
Examples: Cephalaspis, Pteraspis, etc.
Class 2. Cyclostomata (The Circular mouthed fishes:
Gr. cyklos-circularal; stome-mouth).
(1) They occur in the seas and large rivers.
(2) The mouth is circular and jawless. They are also called jawless fishes (not true fishes).
(3) They have 1-16 pairs of gill slits.
(4) Head and brain are poorly developed.
(5) Unpaired fins are present.
(6) Endoskeleton is cartilaginous.
(7) Kidneys are mesonephric.
(8) Stomach is absent.
(9) Respiratory organs are gills.
(10) Heart is two chambered (one auricle and one ventricle).
(11) There are 10 or 8 pairs of cranial nerves.
(12) Lateral line sense organs are present.
(13) Fertilization is external. Life history may include a larva named ammocoete.
Example:
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Petromyzon (Lamprey), Myxine (Hagfish).
Section 2. Gnathostonuita (The Jawed Vertebrates):
Mouth has jaws hence it is named gnathostomata. Embryonic notochord is usually replaced in adult by a vertebral column. Paired fins or limbs are present. Paired nostrils are present. Internal ear has three semicircular canals.
Gnathostomata is divided into two super classes: Pisces and Tetrapoda.
Super class 1. Pisces (Bear Fins):
It includes true fishes and divided into three classes:
Class 1. Placodermi:
Body had an external protective armour of bony scales or plates, e.g. Climatius.
Class 2. Chondrichthyes:
This class includes cartilaginous fishes, e.g. Scoliodon, Torpedo, Chimaera, etc.
Class 3. Osteichthyes:
It includes bony fishes, e.g. Labeo, Hippocampus, etc.
Super class 2. Tetrapoda (Bear Limbs):
Typically all tetrapods (Gk. Tetra – four + podos = foot) possess two pairs of limbs.
