B.Sc. Sem II : - Chordata

 

The animal kingdom is basically divided into two subkingdoms:

(a) Nonchordata – including animals without notochord.

(b) Chordata – This comprises animals having a notochord or chorda dorsalis.While the

Chordata have a notochord at some stage during life; it is not known to exist in the Nonchordata.

• The Chordata is the animal phylum with which everyone is most intimately familiar, since it

includes humans and other vertebrates However, not all chordates are vertebrates.

• All chordates have the following features at some stage in their life (in the case of humans and

many other vertebrates, these features may only be present in the embryos).

• Pharyngeal slits – a series of openings that connect the inside of the throat to the outside of the

“neck”. These are often, but not always, used as gills.

• Dorsal tubular nerve cord – A bundle of nerve fibres which runs down the “back”. It connects

the brain with the lateral muscles and other organs.

• Notochord – a cartilaginous rod running underneath and supporting the nerve cord.

• Post-anal tail – an extension of the body post the anal opening.

ORIGIN OF CHORDATES

It is believed that chordates originated from invertebrates. However, it is difficult to determine from

which invertebrate group of the chordate developed. It is almost constant that chordate ancestors were

soft-bodied animals. Hence, they were not preserved as fossils.

Many theories have been put forward to explain the evolution of chordates; few of them are as follows:

(a) COELENTERATE THEORY:

According to this theory chordates developed from coelenterates. It is believed that radial symmetry coelenteron, cnidoblasts, etc., disappeared and advanced characters developed to give rise to the chordates. This theory infers that chordates might have acquired higher characters independently. This theory is not acceptable.

(b) ANNELID THEORY: 

This theory suggests that the chordates have evolved from an annelid stock, like many chordates the annelids show bilateral symmetry, metamerism, head, lateral coelomate complete digestive tract, closed circulatory system, haemoglobin, etc. The Resemblance is enhanced if an annelid is turned upside down. But the mouth would be dorsal which is unlike that of chordates. Metamerism and appendages of annelids differ in nature from those of the chordates. Bilateral symmetry, head and complete digestive tract occur in other non-chordate phyla also. Coelome is schizocoelic in annelids and enterocoelic in lower chordates. Haemoglobin is dissolved in the plasma in annelids, but it is present in the red blood corpuscles in chordates. The annelid nerve cord is double and ventral in contrast to the single, hollow, dorsal nerve cord of chordates. Some striking differences exist between the annelids and the chordates in their embryology too; hence, it is difficult to accept this theory.

(c) ECHINODERM--HEMICHORDATE THEORY-ORIGIN OF

CHORDATES: This theory infers the origin of chordates, hemichordates and echinoderms from a common ancestor. This theory is based on the following evidence.

1. EMBRYOLOGICAL EVIDENCE: 

Both echinoderms and chordates have enterocoelic coelom, mesoderm and deuterostomous mouth. There is resemblance between the bipinnaria larva of certain echinoderms and the tornaria larva of hemichordates. In echinoderms chordates The central nervous system develops from a dorsal strip of ectoderm.

2. SEROGICAL EVIDENCE:

Similarity exists between the proteins of the body fluid of chordates and echinoderms. Hence the

Chordates and echinoderms are closely related. The radial symmetry of adult echinoderms will

disapprove of their relationship with the bilaterally symmetrical chordates. In echinoderms' radial

symmetry is secondarily developed from a basically bilateral symmetry. Both the primitive and

the early echinoderm larvae show bilateral symmetry.