Phylum Chordata : Part 2

                                       Phylum - Chordata

              Class Amphibia 

• As the name indicates (Gr., Amphi : dual, bios, life), amphibians can live in aquatic as well as terrestrial habitats.
• Most of them have two pairs of limbs.
• Body is divisible into head and trunk. Tail may be present in some.
• The amphibian skin is moist (without scales). The eyes have eyelids.
•  A tympanum represents the ear.
• Alimentary canal, urinary and reproductive tracts open into a common chamber called cloaca hich opens to the exterior.

• Respiration is by gills, lungs and through skin.
• The heart is threechambered (two auricles and one ventricle).
• These are cold-blooded animals.
• Sexes are separate.
• Fertilisation is external.
• They are oviparous and development is indirect.

Earliest amphibians - Ichthyostega

• basically a fish with legs         
• numerous digits on hands, feet
• tail fin with fibrous rays like a fish
• one of the best transitional series in the fossil record is from fish to amphibian

Modern amphibians

• tend to rely on cutaneous (skin) respiration more than lungs, some without lungs
• most still require water for early life stages and/or reproduction
• have 3-chambered heart with pulmonary veins (two-loop system)

Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibia).


d. Class Reptilia

• The class name refers to their creeping or crawling mode of locomotion
• They are mostly terrestrial  animals and their body is covered by dry and cornified skin, epidermal scales or scutes.
• They do not have external ear openings.
• Tympanum represents ear.                                      
  
• Limbs, when present, are two pairs.
• Heart is usually three-chambered, but four-chambered in crocodiles.
• Reptiles are poikilotherms.
• Snakes and lizards shed their scales as skin cast.
• Sexes are separate.
• Fertilisation is internal.
• They are oviparous and development is direct.

Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator). Hemidactylus (Wall lizard), Poisonous snakes – Naja (Cobra), Bangarus (Krait), Vipera (Viper).

e. Class Aves– birds

• The characteristic features of Aves (birds) are the presence of feathers and most of them can fly except flightless birds (e.g., Ostrich).
• They possess beak.                                                
  
• The forelimbs are modified into wings.
• The hind limbs generally have scales and are modified for walking, swimming or clasping the tree branches.
• Skin is dry without glands except the oil gland at the base of the tail.
• 
  
• Endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic).
• The digestive tract of birds has additional chambers, the crop and gizzard.
• Heart is completely fourchambered.
• They are warm-blooded (homoiothermous) animals, i.e., they are able to maintain a constant body temperature.
• Respiration is by lungs. Air sacs connected to lungs supplement respiration.
• Sexes are separate.
• Fertilisation is internal.
• They are oviparous and development is direct.
• feathers – derived scales, made of keratin
• flight skeleton
• bones tend to be hollow (trait shared with some dinosaurs)
• backbone fused – increase stability
• fused collarbones (“wishbone”)
• keeled sternum – increase area for muscle attachment
• have 4-chambered heart with pulmonary veins (2-loop system) – required to be endothermic or “warm- blooded” = increased metabolism for flight
• superefficient respiration – have air sacs that assure that oxygenated air is always going over the lungs
• lay amniotic eggs (like reptiles)

Examples : Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock),

Aptenodytes (Penguin), Neophron (Vulture).


f. Class Mammalia

• hair unique to and defining of mammals
• long filament of mainly dead skin cells filled with keratin
• used for many purposes – sensory, insulation, camouflage, coloration, protection
• mammary glands – milk-producing glands
• have 4-chambered heart with pulmonary veins (2-loop system), allowing them to be endothermic(“warm-blooded”) and homeothermic
• heterodont teeth

•  most have a placenta, a specialized organ that serves as an interface between mother and fetus in the womb (each bloodstream kept separate but in close contact; food, water, oxygen, and antibodies given to fetus, while wastes are carried away) .