ORIGIN OF CHORDATES

ORIGIN OF CHORDATES

CHORDATE ORIGINS: Members of Phylum CHORDATA have four primary characters, they are-

a) A primitive endoskeleton structure called the NOTOCHORD present during the early embryonic life. This rod-like structure composed of a special type of connective tissue which is located along the mid dorsal line where it forms the axis of support for the body.

>>> In some animals, it persists as such throughout the life, while in most chordates it develops into vertebral column.

b) A hollow, dorsal NERVE CORD sometimes during the life. The CNS made up of brain and spinal cord is located dorsally and above the notochord.

     >>>The CNS is a hollow canal from one end of the body to the other end.

c) GILL SLITS connecting to the pharynx are present at some stage of the life.

     >>>Most aquatic chordates respire by gills made up of vascular filament present along the gill slits.

     >>>However in terrestrial chordates, which don’t breathe by gills the gill slits are found only in embryonic life but no vascular filaments are associative with gill slits.

d) Chordates have POST-ANAL TAIL in some stage of life that represents a posterior elongation of the body extending beyond the anal aperture.

>>>The tail is basically a chordate locomotory structure containing the muscle and notochord.

e) Apart from the above four features the chordates also have certain characters common to some other phylum, they are:

>>They are bilaterally symmetrical.

>>Partial metamerism present.

>>They have a true body cavity on coelom lined with mesoderm.

>>They show cephalization i.e. concentration of nerves tissue and presence of special sense organs

>>The blood is flowing anteriorly from the ventrally located heart and pushed to the dorsal side. CHORDATES FROM ANNELIDS AND ARTHOPODS:

The main lines of invertebrate animals show the closest affinity with chordate. Many of the supposed linking similarities between chordates and annelids or arthropods result from ANALOGY rather than HOMOLOGY. Some scientists believe that Annelids and arthropods have the closest similarity because of following reasons:

a) METAMERISM OR METAMERIC SEGMENTAION:

Both group’s shows metamerism but this similarity is superficial.

REASON: The segmentation of this annulates is almost complete division of entire body into rings and units, all organ systems are affected by segmentation. However in Chordates, only dorsal myotomal region is segmented whereas even the ventral part of mesoderm is not segmented.

b) ORIENTATION OF THE CHORDATES:

The two group’s namely Chordates and annulates shows difference in this. The NERVE CORD is dorsal to the duct whereas in annulates it is below the gut and the brain above. Similarly the blood circulation in different directions in two groups and the limbs are also based on different plans. Since scientists like Patten and Gaskell have tried to show relationship with Eurypterids. It is thought that the Eurypterids show some superficial resemblance to the Agnatha of Devonian making the scientists believe that, the type of an annulate could be the possible ancestors of chordates.

c) CLEAVAGE PATTERN:

Development of modern annulates is completely different from chordates. In annulates, the cleavage by which the fertilized eggs gets divided into blastomeres follows a SPIRAL PATTERN in which each blastomere arises a regular way and the future fate of each is exactly defined. In contrast the cleavage in chordates is RADIAL AND IRREGULAR.

d) GASTRULATION: The next stage of development i.e. gastrulation is also different in two groups. In chordates, it occurs by invagination of one side of the ball of cells to form an archenteric cavity communicating with exterior. However in annulates, the cells forming guts migrate inward all around the sphere and later get arranged into a tube that opens to the outside.

e) FATE OF BLASTOFORE:

It is another point of comparison so that in chordates, protochordates and in Echinoderm blastopore becomes the anus while the mouth is a new opening hence these animals come under group DEUTEROSTOMIA. Distinct from non-chordata that belongs to PROTOSTOMIA in which blastopore becomes the mouth.

f) The method of mesoderm and coelom formation are also important i.e. DEUTEROSTOMIA this 3rd layer is produced by separation of endoderm so that coelom is at first continuous with the archenteron and it is said t be an ENTEROCOEL. However in the annulates, the cells separate in various ways to form a mesoderm and a coelom than arises within this solid mass as a SCHOZOCOEL.

Thus in all the above points the chordates differ from all the annulates but resembles Echinoderms and their allies.

Further features also support this relationship. For e.g.: Both have a larva with longitudinal ciliated bands and very different from the TROCHOPHORE LARVA OF ANNULATES which has transverse cilia.

>>>Similarly NERVOUS SYSTEM of annulates consists of a set of ganglionated cords whereas in Echinoderm-like animals it is a diffused sheet of fibres and cells below the epidermis. The nerve chord of the chordate can be easily derived from Echinoderm nervous system and not from the annulates type of nervous system.

>>>A MESODERMAL SKELETON is found in both chordates and echinoderms but absent in annulates.

>>> The mechanism for providing energy liberation also shows systematic differences. For e.g. regeneration of ATP is brought about by Phosphoarginine and its Kinase in the arthropods and molluscs, while Phosphocreatinine and its Kinase bring about the same reaction in Amphioxus and chordate while both this systems operate in echinoderms and urochordates. This may have the advantage; a rich energy source like Creatinine which could be easily synthesized from glycine which could be kept aside for locomotion whereas the rare Arginine could be utilized only for the important histone formation. Early on during evolution it was found creatinine was characteristic of early larva life where arginine of adult life but in neoteny, creatinine became the only substrate for ATP generation.