They are only present during embryonic development in some chordates .Key feature of chordates are:
1) Notochord: Notochord provides skeletal support and develops into the vertebral column in vertebrates.
2) Nerve chord: Dorsal hollow nerve chord develops into the central nervous system, brain spines.
3) Pharyngeal slits: These are the openings in the pharynx that develops into gill arches in bony fish and into the jaw and inner ear in terrestrial animals.
4) Post anal tail: It is a skeletal extension of the posterior end of the body, being absent in humans and apes, although present during embryonic development.
Classification of chordates:
Chordates are classified in following categories:
· They are solitary, colonial, mostly tubiculous, exclusively marine.
· Bilaterally symmetrical.
· Body is soft, fragile, vermiform and divisible into proboscis, collar and trunk.
· Coelom is enterocoelous or true coelom, divisible into protocoel, mesosoel and metacoel.
· Buccal diverticulum earlier considered as notochord present in proboscis.
· Digestive tract is complete, in the form of U- shaped tube.
· Gill slits when present are paired and their numbers are one to numerous.
· Circulatory system is simple and well developed (closed).
· Excretion is by single glomerulus.
· Reproduction is sexual. Sexes are separate or united and gonads are one to several pairs.
· Fertilization is external. Development is indirect through a free swimming tornaria larva. Direct development is also found in some species.
2) Tunicates or urochordates: They are small animals of 1-5 cm long. They are marine animals, either benthos (bottom dwellers) or pelagic (inhabitants of open water), that often form colonies by asexual reproduction. Here notochord restricted to tail, body is covered with tunic or cuticle. Sessile or heart is present.
3) Cephlochordates: Notochord extends to the entire body length with tip anterior to nerve chord. They usually live partially live buried in marine sand and gravel. They have well developed musculature and can swim rapidly by undulating the body.
4) Vertebrates: Notochord extends to the back of well developed head, segments are well developed, heart present. Gill slits function as respiratory devices with the invasion of vertebrates into fresh water and then into the land. There was a shift in means of breathing from gills to lungs.
Characteristics of vertebrates:
Ø Vertebrates are highly cephalized and have well developed organ. They have endoskeleton and closed circulatory system.
Ø Ability to regulate the body temperature.
Ø They have bony endoskeleton which consists of cranium, limb girdles, visceral arches and two pairs of appendages.
Ø Muscles are attached with endoskeleton which helps in locomotion
Ø They have ventral heart with 2-4 chambers.
Ø They have large digestive system with liver, digestive glands and pancreas.
Ø Body plan of vertebrates consist of head, trunk, appendages and post anal tail.
Lets discus about vertebrates in detail:
Vertebrates are classified in following categories:
1) Pisces or Fishes:
Ø They are ectothermic and aquatic vertebrate.
Ø Skin is covered with scales with scales and feathers.
Ø Limbs are modified into fins to facilitate swimming
Ø They have two chambered heart i.e. one auricle and one ventricle.
Ø Fishes don’t have external ears and eyelids.
Ø Lateral line is present on ventral side of body.
Ø Respiration occurs through gills.
Ø Pisces need water for laying eggs.
Ø Cold blooded animals i.e. they regulate or change their body temperature according to the temperature of surrounding
Ø Pisces have vertebral column.
Ø Pisces are classified in three categories:
a) Agnatha: It includes jawless fishes; don’t have scales on their body surface. Eg: Lampreys, Hagfish
b) Chondricthyes: Fishes that have cartilaginous skeleton( flexible skeleton) includes in this category. Eg: Sharks ,Rays
c) Osteichthyes: All fishes that have bony skeleton comes under osteicthyes category.
Eg: Tuna, Bass, Salmon, Trout
Ø Evolved from fully aquatic tetrapods.
Ø Name amphiba is evolved from greek word amphibious meaning” living a double life” because they spend half of their lives on land and half in water.
Ø They are cold blooded animals.
Ø Carnivorous in nature.
Ø Respiration through lungs and gills both, in early stage or larval stage gills are present to respire and in adult stage lungs is used for respiration purpose.
Ø They also have two chambered heart i.e. one auricle and one ventricle.
Ø Many of them keep their skin moist by periodically returning to wet areas.
Ø They also need a water medium to reproduces; otherwise their eggs would dry out.
Ø Eggs of amphibians are covered with jelly like substance i.e. very clear to protect the egg from desiccation.
Ø Most of the amphibians undergo metamorphosis.
Ø Eg: Frog, Salamander, Toads, and Newts.
Ø They are terrestrial animals, found on land.
Ø Most of them are tetrapods animals, walked on four legs.
Ø Pulmonary respiration is present, respiration through gills.
Ø Cold blooded animals
Ø They move by creeping or swimming.
Ø Reptiles have backbone or spinal cord except snakes.
Ø Reptiles have three chambered heart i.e. two auricles and one ventricle, except alligators and crocodiles that have four chambered heart.
Ø Maternal care is present.
Ø Eg: Snake, Turtle, Lizard, Crocodiles.
IV. Aves or Birds:
Ø They can be herbivores or carnivores depending on the species.
Ø Body is covered with feathers which provide insulation from external environment.
Ø They have horny beak and scaly feet.
Ø They are endothermic vertebrate.
Ø They have bipedal locomotion i.e. walk on two legs.
Ø Aves have four chambered heart i.e. two auricles and two ventricles.
Ø Shape of their body is streamlined which facilitates them in fly high in sky
Ø Bones are light weight and hollow which helps aves in flight. Some birds are capable of flight, while others are sedentary or some are flightless like emu and kiwi.
Ø They are keen in vision or have strong vision; sense of smell is highly developed while auditory range is limited.
Ø They are warm blooded i.e. their body temperature does not change with the surrounding temperature or they are able to maintain constant body temperature.
Ø They are oviparous, lay hard shelled eggs which protect them from jerks and further hatch by parents to develop into young ones.
Ø Maternal care is found here.
Ø Eg: Sparrow, Owl, Humming Bird, Penguin
Ø Warm blooded vertebrates.
Ø Have hair and fur on the outer body surface which provides insulation from adverse condition.
Ø They have four chambered heart.
Ø They have sebaceous or sweat glands.
Ø They possess heterodont dentition (different types of teeth like incisors, canines, molars and premolars).
Ø Eg: Bear, Dogs, Whales, Elephant
There are some conspicuous and identifying features of mammals:
Ø They are easily identified by external ears.
Ø Have milk secreting glands or mammary glands to nourish their young ones.
Ø Some mammals are egg laying like platypus or others directly give birth to young ones.
Ø Have hair and fur on the outer body surface.
Ø Internal fertilization present, mammals conceive their young within reproductive tract and mother give birth to their young one.
Ø Monotremes like platypus, spiny anteater are the only mammal that lay eggs
Animals are classified into two class chordates and non- chordates. These classes are further subdivided into phylum. Non chordates are less developed animal than chordates where all physiological organs are not fully developed.