(1) A dorsal hollow or tubular nerve cord. (2) A longitudinal supporting rod-like notochord.
(3) A series of pharyngeal gill slits. These characters may be described in some details. (1) Dorsal tubular nerve cord: Dorsal tubular nerve cord constitutes the central nervous system. In chordates, 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 is no ganglion on the nerve cord but it serves for the integration and coordination of the body activities. Anteriorly, nerve cord forms a cerebral vesicle or brain which is enclosed in a protective bony or cartilaginous cranium. The posterior part of nerve cord becomes the spinal cord which is protected within the vertebral column.
(2) Notochord or chorda dorsalis: The notochord is an elongated rod-like flexible structure extending the entire length of the body beneath the nerve cord. It originates from the endodermal roof of the embryonic archenteron. It is composed of large vacuolated notochord cells, containing a gelatinous matrix and surrounded by an outer fibrous and an inner elastic sheath. (3) Pharyngeal gill slits: All the chordates possess, at some stage of their life history, a series of paired lateral gill clefts or gill slits perforating the pharyngeal wall.
These are variously called as pharyngeal, branchial and visceral clefts or pouches. They serve primarily for respiration and secondarily aids in filter feeding. In protochordates e.g., Branchiostoma 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.