. Mouth is circular without jaws and suctorial. Heart is two chambered and circulation is of closed type. No paired appendages.
Cranium and vertebral column are cartilaginous. Cyclostomes are marine but migrate to fresh waters for spawning (anadromous migration) . After spawning within a few days they die. The larvae (ammocoete) after metamorphosis returns to the ocean.
Examples: Petromyzon (Lamprey) and Myxine (Hag fish) (Figure . ). Figure . Examples of Cyclostomata Lamprey Hag fish fish like coelomate forms with chordate characters such us notochord, dorsal tubular nerve cord and pharyngeal gill slits throughout their life.
Closed type of circulatory system is seen without heart. Excretion is by protonephridia . Sexes are separate, Fertilization is external. Development is indirect and includes a free swimming larva (Figure .
). Example: Branchiostoma (Amphioxus or lancelet) Figure . Example of Cephalochordata - Amphioxus Pharyngeal gill slits Atrial pore Cerebral vesicle Nerve cord Buccal Mouth Gonads Anus Notochord . .
Subphylum-Vertebrata (L . Vertebrus –back bone ) Vertebrates are also called higher chordates and they possess notochord during embryonic stage only. The notochord is replaced by a cartilaginous or bony vertebral column in the adult. Hence all vertebrates are chordates but all chordates are not vertebrates.
Vertebrates possess paired appendages such as fins or limbs. Skin is covered by protective skeleton comprising of scales, feathers, hairs, claws, nails, etc. Respiration is aerobic through gills, skin, buccopharyngeal cavity and lungs. Vertebrates have a ventral muscular heart with two, three or four chambers and kidneys for excretion and osmoregulation.
Subphylum Vertebrata is divided into two divisions, Agnatha and Gnathostomata . Agnatha includes jawless fish-like aquatic vertebrates XI Std Biology-Zoology Chapter- shaped body. Skin is covered by ganoid , cycloid or ctenoid scales . Respiration is by four pairs of filamentous gills and is covered by an operculum on either side.
Air bladder is present with or without a connection to the gut. It helps in gaseous exchange (lung fishes) and for maintaining buoyancy in most of the ray finned fishes. They have a ventrally placed two chambered heart. Excretory organs are mesonephric kidneys and