The main features of chordates are the presence of notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. The body is bilaterally symmetrical, triploblastic, coelomate. Organ system level of organization is present. Closed circulatory system is present. A post anal tail is present.
Phylum Chordata is divided into three subphyla which are as follows:
Subphyla Urochordata and Cephalochordata are usually referred to as protochordates. These are exclusively marine animals. In Urochordata, notochord is present only in larval tail. In Cephalochordata, the notochord extends from head to tail region and is persistent throughout the life.
Common Examples: Urochordata – Ascidia, Salpa, Doliolum; Cephalochordata – Branchiostoma (Amphioxus or Lancelet).
Notochord is present during the embryonic period only. After that, the notochord is replaced by a cartilaginous or bony vertebral column in the adult.
A ventral muscular heart is present as the main organ of the circulatory system. The heart can have two, three or four chambers. Kidneys are present for excretion and osmoregulation. Paired appendages are present for locomotion.
|Notochord present||Notochord absent|
|Pharynx is perforated by gill slits.||Gill slits absent.|
|Heart is ventral.||Heart is dorsal (if present).|
|Post anal tail is present.||Post anal tail is absent.|
Phylum chordata can be divided into two sub-phyla, viz. agnatha and gnathostomata.
There is only one class; Cyclostomata in the sub-phylum Agnatha. Gnathostomata is divided into two super-classes, viz. pisces and tetrapoda. Fins are present in pisces for locomotion. Four limbs are present in tetrapoda for locomotion. Super-class tetrapoda is further divided into four classes, viz. amphibia, reptilia, aves and mammalia.
All the existing members of the class Cyclostomata are ectoparasites on some fishes. The body is elongated. There are 6 -15 pairs of gill slits for respiration. A sucking and circular mouth is present. The body is devoid of scales and paired fins.
Cranium and vertebral column are composed of cartilage. Closed circulatory system is present.
Reproduction: They are marine animals but migrate to freshwater for spawning. They die within a few days of spawning. The larvae return to the ocean after metamorphosis.
Common Examples: Petromyzon (Lamprey) and Myxine (Hagfish).
These are also known as cartilaginous fish because of cartilaginous endoskeleton. These are marine animals. Mouth is ventrally located. Notochord is present throughout the life. Gill slits are separate and operculum (gill cover) is absent. Skin is tough and has minute placoid scales. Teeth are modified placoid scales and are backwardly directed. Jaws are very powerful. These are predaceous animals. Air bladder is absent and hence the animal has to swim constantly to avoid sinking.
There are two chambers in the heart. Some animals of this class have electric organs, e.g. torpedo. Some animals, on the other hand, possess poison sting, e.g. Trygon. They are cold-blooded (poikilothermous) animals.
Reproduction: These are dioecious animals. The pelvic fins in males bear claspers. Fertilization is internal and many of them are viviparous.
Common Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray).
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