Three Fundamental Characters of chordata

 All the chordates possess three outstanding unique characteristics at some stage in their life history.

These three fundamental morphological featuresinclude :

(1) A dorsal hollow or tubular nerve cord

(2) A longitudinal supporting rod-like notochord

(3) A series of pharyngeal gill slits

1. Dorsal hollow nerve cord. The central nervous system of the chordates is present dorsally in the body. It is in the form of a longitudinal, hollow or tubular nerve cord lying just above the notochord and extending lengthwise in the body.The nerve cord or neural tube is derived from the dorsal ectodermal neural plate of the embryo and encloses a cavity or canal called neurocoel. There are no distinct ganglionic enlargements. The nerve cord serves for the integration and coordination of the body activities.

In vertebrates, the anterior region of nerve cord is specialized to form a cerebral vesicle or brain which is enclosed by a protective bony or cartilaginous cranium. The posterior part of nerve cord becomes the spinal cord and protected within the vertebral column.

2, Notochord: The notochord is an elongated rod-like flexible sttuelure extending the length of the body. It is present immediately beneath the nerve cord and jusi above the digestive canal. It originates from the endodermal roof of the embryonic archenteron. Structurally, it is composed of large vacuolated notochordal cells containing a gelatinous matrix and surrounded by an outer fibrous and an inner

elastic sheath The notochord is the prime diagnose feature of the the phylum Chordata which derive its name from it, it serves as a support or internal skeleton and is not to be confused with the nerve cord.Protochordates have a typical notochord. In adultvertebrates, it is surrounded or replaced by the vertebra! column.

3. Pharyngeal gill slits. In all the chordates, at some stage of their life history, a series of paired lateral gill clefts or gill slits perforate through the pharyngeal wall of the gut behind the mouth. These are variously termed as pharyngeal, branchial and visceral clefts or pouches. They serve primarily for the passage of water from the pharynx to outside, thus bathing the gills for respiration. The water current secondarily aids in filter feeding by retaining food particles in the pharynx.

In protochordates (e.g. Bronchia stoma) and lower aquatic vertebrates, the gill slits are functional throughout life. But, in higher vertebrates, they disappear or become modified in the adult with the acquisition of pulmonary respiration.