9.4.1 Fundamental or Basic Characteristics of Chordates
All chordates possess three basic or fundamental characteristics, during some stages or in their whole life that is the notochord, dorsal hollow nervous system, gill slits, and as a fourth character, and sometimes tail is also present.
Notochord is an unjointed solid skeleton, placed above the alimentary canal and below the dorsal body wall, and the central nervous system appears in the embryo of all chordates. In invertebrate chordates, it is present as such throughout life but in the vertebrates, it is replaced into the vertebral column, which is segmented. Serves as an axial endoskeleton and gives support to the body.
Dorsal hollow nervous system: The central nervous system in chordates is dorsally placed, located above the notochord. It is a hollow and fluid-filled and non ganglionated nerve cord.
Pharyngeal Gill slits and gill pouches are paired sets of openings in the pharyngeal region, in aquatic chordates it persists and functions as a respiratory organ but in terrestrial vertebrates it is replaced by Eustachian or auditory tube, Parathyroid, tonsils, and thymus.
9.4.2 Invertebrate Chordates (Acrania)
The term Acrania means "without skull" This division includes animals with the following characteristics: Without brain and cranium (brain box). Jaws, RBC, and paired appendages are absent. Notochord never changed into the vertebral column. This group consists of two subphyla. Subphylum Urochordata (Uro; tail):
In this subphylum, the notochord is present only in the tail region of larva while disappearing in adults. The nerve cord also disappeared in adults, only dorsal ganglion is present. The larvae are free swimming while adults are nonmotile. The body is covered with a tunic (test). therefore also called tunicata e.g., ascidia, herdmania, molgula.
Subphylum Cephalochordata (Cephalo; head):
Notochord runs mid-dorsally throughout the body. Notochord and nerve cord persist throughout life. Example Amphioxus (Branchiostoma).
9.4.3 Vertebrate Chordates (Craniata)
It has only one subphylum known as vertebrates. Vertebrates possess the following characteristics:- Notochord replaced into the vertebral column. The brain and cranium or brain box are present. Paired appendages are present. Teeth and jaws are present. RBC is present. Kidneys are their excretory organs. Tail is also present. Either aquatic or terrestrial. Many are aerial and arboreal.
The vertebrate chordates are divided into two superclasses, Agnatha and Gnathostomata. The Agnatha are without true jaws and no paired appendages, while Gnathostomata has true jaws and paired appendages. Agnatha: It has only one living class Cyclostomata.
9.4.4 Class Cyclostomata
(Cyclo; circular, stoma; mouth, because their mouth is circular).
These fishes are without true jaws and are the most primitive group of living vertebrates. Their body is eel-shaped and scales are absent. The skin is smooth and soft.
Cartilaginous skeleton and suctorial mouth. No paired appendages. Two chambered venous hearts and many aortic arches. A digestive system without a stomach. These animals contain 6-14 pairs of gills for respiration.
Either unisexual (such as lamprey) or bisexual (such as hagfish). Cyclostomes have a single testis or single ovary and are without ducts. Fertilization is external, both direct and indirect development is present. In Lamprey prolonged larval period. Example; Myxini and Lamprey.
0 Comments