Phylum ChordataGENERAL CHARACTERS
The chordate animals at some time in their life history exhibit the following diagnostic characters:
- It is an elastic, solid, skeletal rod lying below the nerve cord and above the alimentary canal.
- It serves as a primitive internal skeleton and acts as a rigid axis.
- It may persist throughout life or it may be replaced partially or completely by a backbone or vertebral column.
- There is a dorsal, hollow, fluid filled nerve cord.
- It is formed by enfolding of a mid-dorsal strip of ectoderm and it generally sinks below the surface.
- It lies above the notochord and outside the coelom.
- It persists throughout life in most chordates, but in a few it degenerates before maturity.
- These are paired openings leading from the Pharynx to the exterior.
- Such gill clefts appear during the development of every chordate, but in many aquatic forms they are lined with vascular lemallae, which forms gills for respiration.
- In terrestrial chordates, which never breath by gills, gill clefts are present during early development but later on, they disappear.
- All the chordates have paired pharyngeal pouches at some stage of their life cycle.
- These extend from laterally from the anterior part of the digestive tract towards the body wall.
- Chordates are triploblastic.
- They are bilaterally symmetrical.
- True coelom is found.
- They are found in almost all the habitats of the World.
The Phylum Chordata is divided into two groups which are:
1. Acraniata (Protochordata)
2. Craniata (Vertebrata)
1. GROUP ACRANIATA (PROTOCHORDATA)
- They are first or simple Chordates.
- Brain box (Cranium) is absent and hence brain is not prominent.
- Notochord does not transform into vertebral column.
a) Sub-Phylum Urochordata (Notochord in tail)
b) Sub-Phylum Cephalochordata (Notochord head to tail)
A) SUB-PHYLUM UROCHORDATA (NOTOCHORD IN TAIL)
- They are also known as “Tunicates” because their body is enclosed in a sac called “Tunic.”
- All members are marine and sessile.
- Body possesses two openings, an incurrent or buccal siphon and an excurrent or Atrial siphon, through these openings exchange of gases and food or waste material take place.
- As a result of “Retrogressive metamorphosis” the larva loses its tail and most of chordate characters and converts into an adult.
B) SUB-PHYLUM CEPHALOCHORDATA (NOTOCHORD FROM HEAD TO TAIL)
- This is a small group of marine animals, body with pointed ends.
- Usually live buried in sand, in shallow water with anterior end protruded out.
- They show all typical chordate characters (hollow dorsal nerve chord, pharyngeal gill slits and notochord).
- Only two genera are present around the world.
2. GROUP CRANIATA (VERTEBRATA)
- In these chordates brain is protected inside a skeletal brain box called “CRANIUM.”
- Also known as “Vertebrates” because notochord is replaced by a vertebral column.
- This group is sub-divided into two sub-phyla, which are as follows:
b) Sub-Phylum Gnathostomata (Mouth with Jaws)
A) SUB-PHYLUM AGNATHA (MOUTH WITHOUT JAWS)
- This is a small group of marine vertebrates also known as “Cyclostomes.”
- Superficially they resemble the fish but lack the jaw so they are often known as “Jawless Fishes.”
- They have rounded suctorial mouth with many rings of teeth.
- Paired fins and scales on body.
- Usually parasitic in nature.
B) SUB-PHYLUM GNATHOSTOMATA (MOUTH WITH JAWS)
- It is a large group of vertebrates with both upper and lower jaw.
- Teeth may be present or absent.
i) Pisces (Fishes)
I) SUPER CLASS PISCES (FISHES)
- This is the largest group of chordates, which includes half of the chordate (25,000 species).
- Study of fishes is called “Ichthyology.”
- Body is streamlined with paired fins and covered over by dermal scales.
i-a) Chondrichthyes (Cartilage Fishes)
i-b) Osteiochthyes (Bony Fishes)
I-A) CLASS CHONDRICHTHYES (CARTILAGE FISHES)
- Alternate name is “Class Elasmobranchi.”
- Usually includes marine fishes with endoskeleton of cartilage (soft bone).
- Skin contains sharp tiny enamel coated denticles called “Placoid Scales,” which form exoskeleton.
- Mouth is ventral in position and tail fin is “Heterocercal.”
- Five exposed gill slits, which are not covered over by a gill cover.
- Common examples are Skates, Sharks, Rays and Scoliodon (Dog Fish)- a small Shark etc.
- Alternate name is “Teleostom,” actually the largest class of chordates.
- Includes marine and fresh water fishes.
- Mouth is present at anterior tip.
- Endoskeleton in these fishes is made up of hard bone while exoskeleton is made up of thin bony plates, which are known as “Cycloid” or “Ctenoid scales.”
- Gills are covered over on each side by a gill cover called “Operculum.”
- An air bladder is present which acts as a hydrostatic organ.
- Tail fin is usually “Homocercal or Diphycercal.”
- Common e.g are Eel, Sea-Horse, Flying Fish, Globe Fish etc
- Zoogeographically important fishes, belonging to group “Dipnoi, included in Class Osteiochthyes.
- Only three living genera.
- They respire by gills and by lungs during drought period (Lungs-Modified air bladder).
- Limited distribution in South America, Africa and Australia.
II) SUPER CLASS TETRAPODA
It includes following classes:
a) Class Amphibia
b) Class Reptilia
c) Class Aves
d) Class Mammalia
A) CLASS AMPHIBIA
- This class includes the animals that came out of the water and established a successful life on land.
- They took advantages of the improved possibilities by remaining close to water, by keeping a soft and moist skin, by developing lungs and by evolving a bony skeleton with a strong vertebral column and four legs.
- They cope with seasonal changes by burrowing during extreme cold and save water by sealing themselves in a mucous envelop on dry land.
- The bony endoskeleton is the main body support.
- The notochord is absorbed during development
- Breathing is mostly by means of skin and also lung, and also by lining of buccal cavity.
- In larva the breathing is mostly by means of external or internal gills.
- The circulatory system shows a three chambered heart, with two atria and one ventricle.
- The amphibians are “Cold Blooded” (Poikilothermic) that is having internal temperature that very with the environment.
- Eggs and sperms are laid in water and fertilization is external.
B) CLASS REPTILIA
The earliest reptiles evolved from the amphibians.
HABIT AND HABITAT
Reptiles are generally well adapted to life on land, in semi-dry, completely dry and even desert habitat.
- All reptiles lay their eggs on land.
- They are cold-blooded animals and are less active during low temperature.
- They possess dry skin covered with epidermal scales.
- In some lizards and crocodiles, small bony plates develop below the epidermal scales.
- The skeleton is built on the same plane as that of amphibians, but is much stronger to support their body weight.
- Respiration takes place exclusively through lungs.
- Heart is three chambered, two auricles and one incompletely divided ventricle. (In Crocodiles, the ventricle is completely divided into two chambers.)
- The excretion takes place through kidneys. The reptiles secrete much of their waste products in form of non-toxic “Uric-Acid.”
- In most reptiles fertilization is internal.
- Eggs are provided with a shell and are laid on land.
- The early development of embryo takes place on the large quantities of yolk and albumin present in the egg.
- Due to the presence of a protective membrane called “AMNION” in the egg, reptiles are included in the “Amniota Group” of Vertebrates.
Alligators, Crocodile, Snake, Turtle and Gecko etc.
C) CLASS AVES (BIRDS)
- Aves have evolved from reptiles.
- As they acquired the capability of true flight they were able to exploit the aerial environment and became the largest class of terrestrial vertebrates.
HABIT AND HABITAT
The birds live from pole to pole in all type of ecological zones. They all breed on land.
FLIGHT AND ADAPTATION
- Feathers differentiate birds from all other vertebrates.
- Feathers originated as extraordinary development of Reptilian scales.
- Instead of growing all over the body and spreading evenly, the feathers grow in definite tracts.
- The feathers play an important role in the thermoregulation of birds. They trap air, which is a bad conductor of heat and so prevent loss of body heat.
- To fly efficiently the birds have reduced their body weight in a variety of ways.
- Many bones become hollow, thin and light.
- Synsacrum and pygostyle are formed by the fusion of vertebrae and give strength to skeleton.
- Birds possess strong muscles to control the use of wing in flight.
- They possess large eyes with well-developed sight.
- The birds communicate with members of their species with sound signals for which the sense of hearing is well developed.
- The great mobility of neck is helpful in feeding, nest building, preening and defence.
- There are developed a number of types of bills according to their feeding habits.
- The digestive system of birds is compact and can accommodate large quantity of food.
- The food is stored for a short period in the crop.
- “Gizzard” possess thick muscular wall with horny lining, small stones swallowed by birds are passed on the gizzard for grinding the food.
- The “Syrinx” or sound-producing organ is found in no other vertebrate except the birds. It is located at the junction between the trachea and the paired bronchi.
- The lungs of birds are small, solid, spongy and slightly distensible. They are in contact with a number of air sacs.
A large number of species of birds exhibit a deep-rooted phenomenon of migration, during which they travel long distances from their summer breeding homes towards areas of warm climate.
SUB-CLASSES OF AVES
There are two main sub-classes of aves, which are:
i) Sub-Class Ratitae (Flightless Birds)
ii) Sub-Class Carinatae (Free-Flying Birds)
I) SUB CLASS RATITAE (FLIGHTLESS BIRDS)
- This sub-class includes modern big sized flight less birds.
- They comparatively have heavy weight and their wings are either vestigial or rudimentary.
- They have a flat sternum without keel.
- Their flight muscles are poorly developed.
- The distribution of these birds is restricted to few areas of the World.
II) SUB-CLASS CARINATAE (FREE FLYING BIRDS)
- In this sub-class modern flying birds are included.
- They are usually small, light weight birds with highly developed wings and feathers with interlocking system.
- They possess sternum with a crest like keel to accommodate the hightly developed pectoral flight muscles.
- The flying birds are distributed all around the World.
D) CLASS MAMMALIA
Early mammals are originated from reptiles. The distinctive characteristic of mammals are at the highest grade of development in animal kingdom.
HABIT AND HABITAT
Mostly terrestrial, a few aquatic.
- They are warm-blooded animals.
- They can maintain a fairly high body temperature and so can successfully survive in colder areas of the world.
- Heat is generated by high metabolic rate of their body and is lost by increasing blood circulation in the skin and evaporation of sweat.
- The mammalian body temperature is maintained at 35˚C-40˚C.
- All mammals possess hair on skin.
- Sweat glands and sebaccous glands are present on skin.
- Mammary glands secrete milk in females.
- External ears (Pinna) are present.
- Teeth are heterodont i.e. not uniform. The different types of teeth are: Incisors, Canine, Premolars, Molars.
- Skull with two occipital condyles is present.
- Lower jaw is composed of single bone on each side.
- Vertebrae are “Gastrocentrous,” composed of three pieces i.e. the centrum and two epiphyses.
- Digits of fore and hind limbs are usually five.
- Cervical (Neck) vertebrae are seven.
- A thick muscular septum “Diaphram” is present between abdomen and thoracic cavity.
- Heart is four-chambered.
- R.B.Cs are non-nucleated.
- Brain with four optic lobes.
- Kidney is metanephrous.
- The stomach is simple sac but rarely complicated.
- Mammals give birth to young ones (Viviparous), which are nourished by parents. Except Prototherians that lay eggs.
- Fertilization is internal.
- Development of eggs occurs in the uterus of female, where the developing embryo develops relationship with mother (Placenta).
- After the birth of the child, the mother nourished her young ones.
Mammals are divided into three sub-class:
1. SUB-CLASS PROTOTHERIA
Includes the egg laying mammals. For example Duck billed, Echidna (Spiny anteater).
2. SUB-CLASS METATHERIA