Phylum-Chordata character and classification.

Phylum — Chordata

Animals belonging to phylum Chordata are shows following fundamental characteristics .

1. Notochord :
During the embryonic development of a chordate there appears a supporting rod called the notochord. It lies dorsal to the alimentary canal and ventral to the nerve cord. In some chordates this structure persists throughout life. In others it is partially or completely replaced by a ‘backbone’. It is made up of separate bony elements or vertebrae. Structurally it is composed of large number of specialized vacuolated cells. It is surrounded by fibrous and elastic sheath. The stiffness of the notochord is due to the turgidity of fluid-filled cells and surrounding connective tissue sheath.

2. Dorsal tubular nerve cord
The nerve cord lies just above the notochord and remains entirely outside the coelom. It is a tubular structure having a small hollow canal running from one end to the other. The dorsal hollow nerve cord persits throughout the adult life of almost all chordates.
3. Gill slits or Pharyngeal clefts 
These are paired lateral clefts leading from the pharynx to the exterior. They are present throughout life in fishes and a few tailed amphibians. In amphibians, like frogs and toads it is found only in the larval stages. In higher vertebrates (reptiles, birds and mammals) they are embryonic and non-functional.
4. Ventral heart
The heart is chambered. It is located ventral to the alimentary canal.
5. Closed blood vascular system
In chordates, the blood passes through a continuous system of tubes namely arteries, capillaries and veins.
6. Hepatic portal system
In chordates, the food laden blood from the digestive tract passes through the capillary net work in the liver, before reaching the heart. Thus the veins originating from the digestive tract as capillaries and ending in the liver again as capillaries constitute the hepatic portal system.
They are bilaterally symmetrical,Triploblastic, coelomate with organ-system level of organisation.

 Phylum Chordata is divided into three subphyla:
Sub phylum 1.  Cephalochordata
Sub phylum 2. Urochordata
Sub phylum 3. Vertebrata.
First three sub phyla are collectively known as Protochordates. Since the members of these sub phyla do not have a cranium or skull they are also referred to as Acrania.

  Sub-phylum 1. Cephalochordata

i. The notochord extends upto anterior end of the body hence this sub-phylum is named so.

ii. The notochord persists throughout life of cephalochordates..

 iii. Pharyngeal gill slits are more numerous and are better developed.

iv. The tail is present throughout life,as in Branchiostoma (Amphioxus).

v. Excretion by protonephridia consisting of solenocyte. 

viA wheel organ of producing currents of water in vestibulae is present.

vi. Paired appendages absent, median fins like dorsal, ventral and caudal, are present.

Eg.Amphioxus

Amphioxus has both ends pointed like lens hence it is commonly called lancelet.

Sub-phylum 2. Urochordata

In Urochordata, notochord is present only in larval tail region.

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.

The notochord is only present in the tail of the larva ( hence named urochardata) and disappears in the adult.

The pharynx is perforated by numerous apertures called stigmata formed by subdivision of the larval gill slits. The stigmata open into an ectoderm lined cavity, the atrium.

The larva (tadpole) undergoes retrogressive metamorphosis, i.e., change from better developed larva to less developed adult, e.g., Herdmania (Sea squirt). 
Dorsal ganglion is found in adult which is formed from dorsal tubular nerve cord of larva.

Vanadocytes are present except Herdmania. Excretion, by neural gland, pyloric gland and nephrocyte.

Circulatory system is open in tunicates.

Sub-phylum 3-Vertebrata or Craniata

i. These are advanced chordates that have cranium or brain box around brain.

ii. Notochord is only present in the embryonic stage, it is replaced by vertebral column or back bone in the adult forms.

 iii. Closed circulatory system with heart 2, 3 or 4 chambered.

iv.lymphatic system present;erythrocytes and haemoglobin are present.

v. A pair of kidneys are present for excretion.

viEndocrine glands are well developed.

viiThey are unisexual; have single pair of gonads.

Examples:Ascidia,Salpa,Doliolum

Division 1: Agnatha or The jawless Vertebrates

They are most primitive of all craniates. The mouth does not possess jaws hence named

Agnatha. Notochord persists throughout life. Vertebral column is represented only by small

imperfect neural arch's over the notochord. They do not have exoskeleton and paired fins or

appendages. They have single nostril and their Internal ear has one or two semi-circular canals. They are cold blooded.

Division 2 : Gnathostomata or The jawed vertebrates.

It includes advanced vertebrates in which the embryonic notochord is usually replaced in adults by avertebral column. Paired fins or limbs are present. Mouth has developed jaws hence it is namedgnathostomata. Paired nostrils are present and internal ear has three semicircular canals.Limb girdles present.