Phylum chordata, Biology

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   

Posted Date: 10/4/2012 3:00:40 AM | Location : United States







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