The word Cordata is formed from the Latin word chorda meaning chord or notocord and ata meaning to bear. Organisms that have rodlike and elastic notochords and hollow nerve cords throughout life or at any stage of life are called chordata. Chordates make up about 3-5% of the world’s population. The number of non-cordata is 68,500 (Hiclman, 2008). Scientist Balfour (1880) created the order Chordata with animals having notochords. Bateson first named the episode.
Characteristics of the chordata phase
1. Vertebral Cord: Chordates have a notochord in embryo or throughout life. In advanced animals the notochord is replaced by the spinal cord.
2. Brain: Chordates have nerve cords. The anterior end of the nerve cord forms the brain.
3. Symmetry: The posterior part of the nerve cord of Chordata animals forms the symphysis.
4. Pharyngeal gill openings: Chordates have pharyngeal gill openings at any stage of life or throughout life.
5. Heart: Cordata animals have a well-formed heart. The heart is always located at the apex of the intestine.
6. Endostyle: Chordata animals have endostyle. The endostyle later transforms into the thyroid gland.
7. Digestive system: Chordata have complete digestive system. It contains intracellular and extracellular digestive glands.
8. Tail: Cordatas have a post-anal segment elastic tail. Sometimes the tail can get lost during the transition.
9. Circulatory system: Chordates have closed circulatory system. It is made up of blood, blood vessels and blood vessels. It has hepatic portal system.
10. Excretory system: The main excretory system of chordates consists of a pair of kidneys. Through this, nitrogenous wastes are removed from the body.
11. Body skin: The body skin of Chordata animals consists of epidermis (derived from ectoderm) and dermis (derived from mesoderm).
12. Endoskeleton: The endoskeleton of chordates is made up of bone or cartilage.
13. Eyes: Cordatas have simple eyes. It originates from the brain.
14. Legs: Vertebrate animals have two pairs of legs.
15. Segmentation: Segmentation of chordates is limited to body wall, brain and tail.
16. Genus: Chordata are unisexual. They have sex. Asexual reproduction is absent.
17. Osmoregulation: Osmoregulation of chordates occurs through the kidney.
18. Silome: True silome is present in the body of chordates. Surrounding the heart is the pericardial cavity.
[key words to remember features: spinal cord, brain, mesentery, pharyngeal pharynx, endostyle, tail]
Urochordata
The term Urochordata is derived from the Greek word oura meaning tail and chorda meaning chord. Animals which in the larval stage have a notochord in the tail and the body is covered by a test or tunic are called Urochordata. These are tail cordata. These are commonly called tunicates. So named because their bodies are covered with cellulose called tunic or testa. Some species of this subspecies are called sea fountains because they forcefully siphon water. Tunicates feed on suspension feeders (small food). Scientist Lamarck named them Tunicata. Their number of species is 2,804.
Characteristics of Urochordata
1. A nortochord is absent in mutants. The larval stage has a notochord (in the tail).
2. The body is covered by a tunic or test. Tunic is made up of cellulose called Tunisian.
3. The metamorphosis adheres to submerged objects by adhesive glands located on the head. That is, living a still life.
4. They are marine and live singly or in colonies.
5. Some animals forcefully draw water with a siphon, so they are called sea fountains or tunicates.
6. The larval stage consists of notochord, nerve cord and phloem. The larva is the free or active stage.
7. The pharynx is like a large sac.
8. Their bodies contain chemicals called didemnin. Didemenin is used in the treatment of cancer.
9. The circulatory system is free, the heart cycle reverses peristalsis (blood flow reverses direction each time).
10. They are bisexual animals. The life cycle consists of tadpole larvae.
11. Larval metamorphosis or retrogressive metamorphosis occurs. That is, advanced features are lost and undeveloped features are gained.
[Key words to remember features: notochord, tunic, testa, fountain, nerve cord, pharynx, pharyngeal sac, didemnin, reverse peristalsis, tadpole, retrogressive metamorphosis]
Classification of Eurochordata
Eurochordata are divided into three classes. The classes are Ascidaceae, Larvaceae and Thalaceae.
1. Ascidiacea: The number of species in Ascidiacea class is 2,860.
(i) They are brightly colored marine animals. The shape of the body is cylindrical or sac-like.
(ii) They are solitary or colonial, stationary or free moving tunicates. Members of the colony are connected to each other by stolons.
(iii) They are known as ascidians, tunicates and marine squits.
(iv) They are called sea fountains.
(v) The body is covered by a soft and leathery tunic.
(vi) The free end of the body consists of external and internal siphons.
(vii) Has numerous pharyngeal florets. The alimentary canal is T-shaped.
(viii) A metamorphosis does not have a tail.
(ix) The notochord disappears in the mature state. They live in shallow water.
Examples: Ascidia mentula, Molgula oculata, Ciona intestinalis, Herdmania momus.
2. Larvacea: Larvacea class has 70 species.
(i) They are small, elongated and much curved frog-shaped animals.
(ii) Complete animals have tail and notochord. The tail is thin and flat.
(iii) Pelagic animals live at the top of the water and benthic animals live at the bottom.
(iv) They have notochord, dorsal nerve cord and smooth muscle in their tail.
(v) They have two gill openings in the pharynx. However, atriopore does not exist.
(vi) Body covering is thin, transparent and temporary.
(vii) Body siphon absent.
(viii) Life cycle of tadpole larvae.
Examples: Oikopleura dioica, Bathochordaeus stygius, Appendicularia sicula.
3. Thaliacea: Number of species of Thaliacea class is 70.
(i) They are solitary or colonial and free floating tunicates.
(ii) Their bodies are lemon or barrel shaped. In the mature state, the body does not have a tail.
(iii) Their body is covered by a thin and transparent tunic.
(iv) Body wall consists of circular muscle rings.
(v) present at the opposite ends of the orifice and atriopo.
(vi) Body has external siphon at one end and internal siphon at other end.
(vii) Many have light-scattering luminous organs in their bodies. Luminous organ emits bright light at night.
(viii) Polymorphism is seen among them. There is no tail.
Examples: Doliolum rarum, Salpa maxima, Pyrosoma atlanticum.
Retrogressive
The process by which organisms lose advanced traits and acquire undeveloped traits is called retrogressive transformation. Metamorphosis occurs in the larval stage of the organism. Early stage larvae are active, inedible and pelagic. They have axial notochord, dorsal neural tube and sense organs. These features are lost during metamorphosis. There are three types of transfiguration. Antecedent changes, progressive changes and molecular changes.
Tunic
The body covering of urochordates is called tunic. It is transparent or semi-transparent and leathery. It is composed of cellulose called Tunisian. Tunisian is an analogue of plant cellulose. For example, Ascidia mentula
Cephalochordata
The word Cephalochordata is formed from the Greek word Kephale meaning head and chorda meaning chord. They are commonly called lancelets. First they are described as Amphioxus (amphi = both means both ends, ends = oxys means sharp). Later, according to the law of precedence, they were named Branchiostomata (branchia = gill, stoma = mouth). Cephalochordates are called invertebrate chordates. In 1774 scientist Peter Simon Pallas (1774) described Branchiostoma lanceolatum. Their number of species is 33.
Characteristics of Cephalochordata
1. Body long, slender, laterally compressed, transparent, boat-shaped and pointed at both ends.
2. Anterior to the body is an oral hood and oral cirri.
3. From the front to the back of the body there is a permanent notochord and nerve cord.
4. The pharynx has numerous gill openings. The gills open into the atrium.
5. At the tip of the body are atriopores.
6. The body is 3-7 cm long and has 60 pairs of myotome muscles on both sides of the body.
7. They look a lot like Ballam Aga. That is why they are nicknamed Bhallaka animals.
8. Their pharynx contains numerous atria and atriopores.
9. They are excreted.
10. They are called lancelets in English.
11. They do not have a clear head.
12. The circulatory system is closed and developed. Hepatic portal system is present.
13. The main excretory organ is protonephridia. Protonephridia contain solenocytes cells.
[KEY WORDS TO REMEMBER CHARACTERISTICS: boat, oral hood, oral cirri, notochord, nerve cord, pharynx, myotome muscle, excreta, lanceolate, protonephridia]
Some animals of Cephalochordata
Branchiostoma lanceolatum (Amphioxus)
Branchiostoma belcheri
Branchiostoma floridae
Asymmetron inferum.
Oral hood
In cephalochordates, the boat-like structure around the mouth is called the oral hood. The facial metapleural fold expands to form the oral hood. For example, Branchiostoma.
Oral cirri
The oral hood of cephalochordate animals has narrow thread-like structures called oral cirri. Oral cirri hang around the mouth. It is a sensory organ. It acts as a filter. Such as Branchiostoma
Endostyle
A pair of mucus-secreting folds in the pharynx of urochordate and cephalochordate animals is called the endostyle. Endostyle is a parallel longitudinal fold. It is located at the base of the pharynx. In vertebrates, the endostyle transforms into a thyroid gland. For example – Branchiostoma.
Vertebrata
The word Vertebrata is derived from the Latin word vertebratus meaning having a backbone. Animals in which the embryonic notochord is fully replaced by bony or cartilaginous vertebrae are called Vertebrata. Vertebrata or vertebrates are known as higher order animals. Because their brain is located in the cranium, their other name is craniata. Their species number is 66,178 (IUCN, 2014).
Vertebrates (fish) have been in the ocean for over 150 million years. Hands and feet evolved in a group of vertebrates about 365 million years ago. As a result, they qualify as amphibians to live in water and on land. Over time, the descendants of amphibians adapted and evolved into reptiles, birds and mammals.
Characteristics of Vertebrata
1. Their notochord is replaced by a spinal cord.
2. The anterior part of the hollow nerve forms the brain and the posterior part the spinal cord.
3. They have 5-15 pairs of pharyngeal florets.
4. Lateral pairs of fins or feet act as their locomotion organs.
5. Blood circulatory system is closed type. It is composed of a muscular heart, blood vessels and red blood (containing hemoglobin).
6. Body skin consists of the epidermis (derived from the ectoderm) and the dermis (derived from the mesoderm).
7. 2-4 chambered heart and blood vessels.
8. The brain is protected by the cortex. Brain contains 10-12 pairs of carotid nerves.
9. There is an endoskeleton made of bone or cartilage. Arises from the intraskeletal neural crest.
10. Skin glands, feathers, scales, nails, claws, hairs, horns, hooves etc. exist in the body.
11. There are different types of endocrine glands in the body.
12. Nutrition is complete; Consists of muscular alimentary canal, liver and pancreas.
13. There are well-developed kidneys for excretion and osmoregulation.
[Key words to remember features: spinal cord, brain, mesentery, pharynx, feet, fins, closed circulatory system, carotid nerve, endoskeleton, excretion, osmoregulation]
The suborder Vertebrata is divided into 2 suborders. Cyclostomata or Agnatha and Nathostomata