NEET 2020: Animal Kingdom: Phylum Chordata Study Notes

Phylum Chordata is important sections in the Animal Kingdom unit which are mostly asked every year in NEET, AIIMS, and JIPMER exams. Go through this article to know about SUBPHYLUM UROCHORDATA, CEPHALOCHORDATA, AND VERTEBRATA. Let's start with the introduction of Animal Kingdom and its classification.

Phylum Chordata Notes for NEET

KINGDOM ANIMALIA: PHYLUM CHORDATA- SUBPHYLUM UROCHORDATA, CEPHALOCHORDATA, AND VERTEBRATA

INTRODUCTION

The animals in the phylum Chordata reflect the following features, that are either present through the life cycle or only during the embryonic stage (Fig.1):

1. Notochord: It is the defining feature of the chordates. It is a dorsal, solid and stiff elongated rod-like structure present between the alimentary canal tube and the neural tube. It provides essential support to the development of the neural tube. Invertebrates, it is replaced with the vertebral column, so it appears only during the embryonic stage. In other chordates, it persists throughout their life cycle.
2. Nerve Cord: The presence of dorsally located and hollow nerve cord or neural tube is another chordate feature. It is ectodermal in origin and it forms the brain and the spinal cord.
3. Pharyngeal Gill Slits: These are the openings located at the posterior region of the mouth. In the vertebrate fishes, these gill slits act as gill support. In terrestrial chordates, these are seen only during the embryonic stage as in adults they develop jaw and ear bones.
4. Post-anal Tail: It is a posterior extension of the body. It is highly muscular and serves as locomotory organs in aquatic chordates. In some terrestrial chordates, it serves as a balancing In humans, it is seen only during the embryonic stage.

DIFFERENCE BETWEEN NON-CHORDATES AND CHORDATES

CLASSIFICATION OF PHYLUM CHORDATA

SUBPHYLUM UROCHORDATA

GENERAL FEATURES

• These are also called the ‘tunicates’ due to the presence of a tunic made of a cellulose-like substance called tunicin that covers their body.
• Their larval stage shows all the characteristic features of the chordates. The adult form has the pharyngeal gill slits only. Hence, the larva is said to go retrogressive metamorphosis, that is the advance larva forms a less developed adult.
• These are the mostly sedentary habit.
• These are mostly bisexual, that is the male sex organs and female sex organs are present within the same organism. The fertilization is internal and the larva hatch from the eggs inside the body of the adult.
• The incurrent siphon takes in the water and it is filtered by the pharyngeal gill slits. The food is transferred to the intestine and the anus passes the undigested food via the excurrent siphon.
• Examples of Urochordates include Herdmania, Ascidia, Ciona, Salpa, Botryllus, Pyrosoma
• Herdmania (sea squirt) has vanadium in the blood which imparts green
• Botryllus is a colonial urochordate.
• Pyrosoma is bioluminescent.

SUBPHYLUM CEPHALOCHORDATA

GENERAL FEATURES

• These animals are characterised with the adult stage having a notochord, dorsal hollow nerve cord, pharyngeal gill slits, and a post-anal tail.
• The notochord extends to form the head. True brain is absent.
• The fossil Pikaia reflect the presence of cephalochordates for more than 500 years.
• The lancelets are the present cephalochordates that have blade-like shape.
• These are usually filter feeders.
• Examples include Branchiostoma, Amphioxus
• Amphioxus has numerous light-sensitive eye-spots.

SUBPHYLUM VERTEBRATA

GENERAL FEATURES

• These are the advance chordates with a cranium covering the brain.
• The notochord is seen in embryonic stage only. It is replaced with the cartilaginous or bony vertebral column.
• The epidermis forms the exoskeleton that can have the scales, feathers or hair.
• The striated, unstriated and cardiac muscles are present.
• Cartilage and bones form the endoskeleton.
• Heart is ventral in location and can have two, three, and four chambers.
• The circulatory system is closed.
• Gills, lungs, skin and buccopharyngeal cavity form the respiratory organs.
• A pair of kidneys performs excretion and osmoregulation.
• The brain and spinal cord are the organs of the central nervous system. The peripheral nervous system has both the somatic and autonomic nervous system.
• Eyes, ears, tongue, nose are the sense organs.

DIVISION AGNATHA

GENERAL FEATURES:

• These are the jawless vertebrates.
• The notochord exists throughout the life cycle.
• There is a single nostril and one or two semi-circular canals in the internal ear.

CLASSIFICATION

DIVISION GNATHOSTOMATA

GENERAL FEATURES:

• These are jawed vertebrates.
• The notochord in the embryo is replaced by the vertebral column in adults.
• Fins and nostrils are paired.
• Three semi-circular canals are present in the ear.
• This division is subdivided into two Superclasses: Pisces and Tetrapoda

SUPER CLASS PISCES: The Fishes

GENERAL FEATURES:

• The pectoral and pelvic fins represent the paired appendages.
• Respiration occurs through the gills.
• Body is divided into head, trunk and tail.
• The head shows eyes with the nictitating membrane, two ears and nostrils.
• The gills are covered with the operculum.
• The caudal or tail fin helps in swimming.
• These are unisexual fishes.
• These are classified into:

1. Class Chondrichthyes
2. Class Osteichthyes

CLASS CHONDRICHTHYES

GENERAL FEATURES

• These fishes have the cartilaginous endoskeleton.
• Caudal fin is heterocercal.
• These are poikilothermic, that is, cannot regulate the internal body temperature.
• These are generally carnivorous.
• The skin bears the tough placoid scales.
• Gills are not covered with an operculum (except Chimaeras)
• The heart is two-chambered with sinus venosus and conus arteriosus.
• Mesonephric kidneys that excrete ammonia.
• 10 pairs of cranial nerves.
• The brain has a large olfactory lobe and cerebellum as compared to the bony fishes.
• The air bladder is absent.
• The oviduct is called Mullerian duct. Fertilization is internal.
• The cloaca is present.
• These can be oviparous (egg-laying) or ovoviviparous (hatching of egg in mothers’ body)

EXAMPLES:

1. Scoliodon (Dogfish): These fishes have characteristic pores on the upper and lower surface of the head called ampullary pores that open into the ampulla called Ampulla of Lorenzini that helps in sensing temperature. The claspers in males help in copulation. It is ovoviviparous. It is a good source of cod oil.
2. Pristis (Sawfish): The head is saw-like rostrum bearing strong teeth called denticles. It is ovoviviparous.
3. Torpedo (Electric Ray): These generate electric currents from the electric organs that are located on the dorsal side of the trunk. These electric organs are supplied with cranial nerves.

CLASS OSTEICHTHYES

GENERAL FEATURES:

• The endoskeleton is cartilaginous initially during the embryonic development. The adults have the bony endoskeleton.
• These are poikilothermic.
• The caudal fin is homocercal.
• The body shows cycloid, ctenoid and ganoid scales.
• Four pairs of gills are present that are covered with an operculum.
• The air bladder is present that allows them to maintain balance.
• The heart is two-chambered with one atrium and one ventricle. The sinus venosus and conus arteriosus are well developed. The lungfishes usually have three-chambered
• Oval and nucleated red blood cells are present.
• Mesonephric kidneys are the excretory organs. These are ammonotelic animals.
• 10 pairs of cranial nerves are present.
• The brain shows comparatively smaller olfactory lobes and cerebellum.
• Fertilization is external. These can be oviparous or ovoviviparous.
• The cloaca is absent.

EXAMPLES:

1. Exocoetus (Flying Fish): It shows leaps in the air as high as 6 meters. These have wings-like pectoral fins. It is a source of food in many regions.
2. Hippocampus (Sea Horse): The head and the neck resemble that of the horse. These swim in an upright manner. The sexual dimorphism is marked in these fishes. The male has a brood pouch in which the female lays the eggs, so the males show parental care. Their skin is widely used in the preparation of ornaments.
3. Labeo rohita (Rohu): These fishes are known for the delicious fish meat. They are found in clear rivers and streams. These are herbivorous or planktivorous in diet. These have barbles present near the mouth.
4. Catla (Theila): These are freshwater fishes and are the source of food in many regions of India. The barbles are absent.
5. Clarias (Catfish): These have well-developed barbles but the scales are absent. These are carnivorous with well-developed teeth.
6. Pterophyllum altum (Angel Fish): These are freshwater fishes with a wide and triangular dorsal fin, hence the name phyllon (leaf-like).

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