SUB-PHYLUM VERTEBRATA is further divided into two super-classes.
AGNATHA, AND GNATHOSTOMATA.
SUPER-CLASS 1. AGNATHA: (A = Without; Gnathos = Jaw).
Earlier vertebrates are included in it They do not show jaws, and paired appendages. Nostril is median and single.
Ex: Petromyzon, Myxin.
SUPER-CLASS 2. GNATHOSTOMATA: (GNATHOS = JAW)
They show true jaws and paired appendages. This group includes the remaining vertebrates from fishes to mammals.
This is divided into 2 important groups. Pisces and Tetrapods.
They include 7 Classes.
CLASS 1. PLACODERMI: The Placodermi are the earliest known Gnathostomes They flourished from the early Devonian to Penman periods of the Paleozoic era and became extinct in the beginning of the Mesozoic era.
CLASS 2. CHONDRLCHTHYES: (Chondro = cartilage; Ichthy = fish). These fishes show cartilage skeleton. Five pairs of gill-slits are present They are not covered b operculum. Spiral valve is present in the Intestine. Air bladder is absent Placoid scales are present Many of them are viviparous.
CLASS 3. OSTELCHTHYES: (Osteo = bone; Ichthy = fish). These fishes skeleton is bony. 4 pairs of gills are present Gills are protected by operculum. Claspers are not present Air bladder is present except in some teleosts.
These 3 classes are Included in the Group Fishes. Some authors call it as Super class Pisces.
CLASS 4. AMPHIBIA: (amphi = two; bios = life). They show amphibious life. Exoskeleton is absent Pentadactyl imbs present. Gills are present either in the embryonic condition or throughout the life. In adult condition lungs are present. Heart is three chambered . Portal circulation is well developed. They lay eggs in water
Ex : Necturus, Rana.
CLASS 5. REPTILLA: (Reptilis = creeping). These are first true terrestrial vertebrates. Lungs are respiratory organs. Skin is protected by epidermal scales or by dermal bony plates. Poikilo-thermous or ectothermal animals. Eggs possess a large amount of yolk. During development extra embryonic membranes are formed.
Ex: Urornastix, Crocodile, Calotes.
CLASS 6. AVE: (Aves = Birds). These are highly specialized vertebrates adopted to aerial mode of life.
Fore-limbs are modified into wings. Exoskeleton is in the form of feathers. These are all homoeothermic i.e., temperature never varies according to the environment. Hind limbs are provided with 4 toes. Heart is four chambered. Only right aortic arch is present. Teeth are not present in modem birds. They lay big megalecithal egg.
CLASS 7. MAMMALIA: (Mammaha = the breast). These are highly specialized vertebrates Body is covered by hairs. In the females mammary glands are present for giving milk to the young Only left aortic arch is seen These are homiothermic animals Sweat glands are present Four chambered heart is seen In many mammals the foetus is in the mothers uterus and is connected by placental stalk.
Ex: Cat, Dog etc.
These 4 classes are included the Group TETRAPODS