PHYLUM CHORDATA
Definition and Introduction
Bilateral and deuterostomial eucoelomate eumetazoa, basically possessing ,in the embryo or throughout life , a flexible, but firm and supporting skeletal rod, called notochord (Gr. Chorda= string).
About 55,000 existing and 25,000 extinct chordate species known . Existing species inhabit land ,water or air throughout the world. Fishes, frogs lizards, snakes, birds, rats, domestic animals man. Etc. Are a few examples of common chordates .Generally chordates are relatively larger and better organized than other metazoan ,i.e. nonchordates.
Chordates are sharply distinguished from nonchordates by three fundamental diagnostic characteristics
1.Notochord; It is a supporting axial endoskeletal rod, forming from embryonic mesendoderm and extending in mid dorsal axis between nerve cord and alimentary canal. It consists of a series of large , vacuolated cells enclosed in a single or double layered sheath of connective tissues .It persists throughout life in some primitive chordates but ,in majority , it occur only during embryonic stages being replaced in adult by a mesodermal. Cartilaginous or body vertebral column.
2.Dorsal tubular central system; Some higher nonchordates (annelids and arthropods ) possess a double and solid, midventral nerve cord beneath alimentary canalo. Chordates ,on the contrary, always possess a single and hollow , tube like nerve cord , extending along middorsal line above alimentary canal and notochord. It develops from embryonic ectoderm. Its cavity is called neurocoel .In most chordates. It anterior part enlarges to form a brain , while the remaining part forms the spinal cord, both together form the central nervous system ( CNS).
3.Pharyngeal gill clefts:; Paired slits, called gill clefts, form in lateral walls of embryonic pharynx in all chordates primarily for respiration, These form in a peculiar manner finger like hollow pouches grow out form pharyngeal wall and meet with corresponding in pocketings of body wall. Later the intervening walls dissolve, forming the clefts which obviously connect pharyngeal cavity with the d exterior ,In lower aquatic chordates gill clefts persist for respiration througout life, in higher, terrestrial chordates, they close during later embryonic development.
In addition to the above three diagnostic characters, the following characteristics also distinguish chordates from nonchordates.
4. Ventral heart: Unlike nonchordates chordates possess a distinct, muscular heart in ventral side, mostly enclosed in a sac like pericardium whose pericardial cavity is part of the coelom.
5.Blood vascular system: When blood flows in fixed direction within well formed vessels , the system is called a closed vascular system ,Chordates and some higher nonchordates such as earthworms, possess a closed vascular system but in chordate, it is much better developed. In contrast to nonchordates, the blood flows in chordates from before backwards in the main dorsal vessel and from behind forwards in the main ventral vessel. Moreover ,all chordates possess a hepatic portal system in which a heoatic portal vein. Collecting blood from alimentary canal , breaks into capillaries within the liver, instead of running straight to the heart.
6.Red blood corpuscles : A red respiratory pigment, haemoglobin is present in blood of chordates and some nonchordates .In chordates it is always contained in special blood , cells called red blood corpuscles (RBCS) or erythrocytes, which in the nonchordates , it is dissolved in blood plasma.
7.Postanal tail : The posterior part of body is thin and tail like in many higher metazoans .In nonchordates , the tail is an extension of the main , hollow body , with anus at its tip. Contrary to his the of chordates chordates is a solid and muscular part of body behind the trunk. It is characterized by metameric segmentation and endo skeleton. It develops in embryos of all chordates and persists throughout life in most. It is primarily locomotory in function