Phylum Chordata - Characteristics, Classification And Examples

What is Phylum Chordata?

Chordata is the phylum of the animal kingdom that includes a large number of animal species, including humans. Phylum Chordata animals include all animals that possess a notochord at least at one point in their lifetime.

Characteristics Of Phylum Chordata

Notochord

Presence of longitudinal, cartilaginous rod called Notochord. It is running between the nerve cord and the digestive tract. It acts as a support for the nerve cord. In the vertebrates, it is replaced by the vertebral column after the embryonic stage.

Dorsal nerve cord

The dorsal nerve cord runs along the “dorsal” and divided into the brain and the spinal cord.

Pharyngeal slits

They are the openings that allow the entry of water through the mouth without entering the digestive system viz. they connect the mouth and throat. Present at least one stage of life history.

These slits are used by invertebrate chordates for filter feeding. Water enters the mouth, filtering food particles as water flows out of the pharyngeal cavities. In aquatic vertebrates, such as fish, the pharyngeal slits are eventually converted into gill bases or jaw bases (as in jawed fish).

Post-Anal tail

Postanal tail is present. It is an extension of the body to the anus. In chordates, the tail contains skeletal muscles that help in the extinction of species such as fish.

Body

The body is bilaterally symmetrical, triploblastic, and metamerically segmented.
Body with true coelom.

Circulatory system

The circulatory system is the closed type with a ventral heart. With distinct arteries and veins.

Other

The body of the phylum Chordata is supported by a live endoskeleton.

The body has an organ system level of organization.

Excretion by the pronephric, mesonephric, or metanephric kidney.

Respiration by skin, gills, or lungs.

The digestive system is complete with separate digestive glands.

Sexual organs are separate, gonads with gono-ducts. There is no asexual reproduction.

Classification Of Phylum Chordata

Phylum Chordata is classified into three subphyla namely,

Urochordata
Cephalochordata
Vertebrata

Hemichordata

Hemichordata is Exclusively Marine, Solitary or colonial, mostly tubicolous.
The body of hemichordates is soft, fragile, vermiform unsegmented, bilaterally symmetrical, and triploblastic.
The body of this subphyla is divisible- proboscis, collar & trunk.

Coelom enterocoelous is divisible into – protocoel, mesocoel & metacoel, corresponding to 3 body regions.
For-gut gives out a hallow buccal diverticulum into proboscis earlier considered as the notochord.
Digestive tract is complete & straight /U-shaped. Dorso-lateral Gill-slits when present, one to several pairs. Ciliary filter feeder

Circulatory system – simple and open, a dorsal contractile heart vesicle, two longitudinal vessels and one dorsal and one ventral, interconnected by lateral sinuses and vessels.
Nervous system -primitive having intra-epidermal plexus. Two nerve cords, dorsal collar cord hollow.
Excretion- this process takes place by a single glomerulus in proboscis connected to blood vessels.

Reproduction happens mostly sexually, gonads are separate/united, gonad 1 to several.
The process of fertilization takes place externally, development can be direct/ indirect through free-swimming Tornaria larva.

Urochordata

Urochordates are mostly marine cosmopolitan, known as sea squirts.

Solitary/colonial is Fixed or free-swimming and pelagic.
The body is covered by cuticular tunic/test in the adult stage, composed of tunicin (like cellulose).
Anterior branchial aperture and dorsal atrial aperture are commonly present,

The coelom is absent in the subphyla Urochordata, the pericardial cavity is not coelomic.
The atrium is lined by ectoderm and this opens outsides by atriopore.
The notochord is present in larval stages only.

The pharynx is large with two/several pairs of gill-slits, endostyle present, ciliary feeding.
The blood vascular system is open type, heart-ventral, simple, and tubular, blood is without respiratory pigments.
The dorsal tubular nerve cord is present in larval forms, degenerates in the form of the small ganglion in adults.

Excretion takes place by the neural gland, nephrocytes, and pyloric gland, Sexes- united i.e. hermaphrodite.
The fertilization process is external and cross.
Development – Development is indirect with tadpole larva having chordate characters.

Cephalochordata

The body is fish-like and it is useful for burrowing and swimming.
subphylum Cephalochordata has a distinct head and tail.
Appendages are absent in Cephalochordata.

Dorsal, caudal and ventral fins are present in the subphylum Cephalochordata.
The body of the wall shows one-cell thick, non-ciliated epidermis, dermis, connective tissue, striated muscle, and parietal peritoneum.
Enterocoelic coelom is present. However, reduced in the pharyngeal region by an atrium.

The notochord of Cephalochordata extends from the anterior end to the posterior end.
The alimentary canal in Cephalochordata is long. It includes a large pharynx with many gill-slits ciliary modes of feeding are developed.
The circulatory system is closed.

Heart and respiratory pigments are absent.
A hepatic portal system is present.
The dorsal tubular nerve cord extends throughout the length.

The brain is not differentiated.
Two pairs of cerebral and several pairs of spinal nerves are present.
The excretory system shows paired protonephridia with solenocytes.

Sexes are separate. Gonads are metamerically arranged and without gonoducts. No Asexual reproduction.
The process of Fertilization takes place externally.

Examples –

Balanoglossus

They are solitary, free-swimming, or burrowing commonly called acorn or tongue worm

The body of balanoglossus is worm-like elongated, vermiform, and stalk is absent.

The proboscis is cylindrical and tapering.

The collar is short and without ciliated arms.

The trunk is long.

Gut straight, mouth and anus at opposite ends and Filter feeder.

Several pairs of U-shaped gill-slits.

Sexes are separate, gonads are numerous and sac-like.

The development in balanoglossus includes tornaria larva in some. They lack Asexual reproduction.

About 70 enteropneust species have been identified.

Class – Pterobranchia

Solitary or colonial, sessile and tubicolous, living inside secreted chitinous tubes.
The body of class pterobranchia is short, compact, and with a stalk for attachment.
The structure of the Proboscis is shield or disc-shaped.

Collar bears are ventrally the mouth and on the dorsal side possess one to nine pairs of lophophore arms bearing tentacles.
The trunk is short and sac-like structured.
The gut is U-shaped, with or without pharyngeal slits, anus dorsal lying near the mouth.

Gills slits are 1 paired or absent, never U-shaped.
Sexes are separate or united, gonads 1 or 2.
Development is direct or with larval stages. Asexual reproduction takes place by the budding method.

Class – Larvacea

Class Larvacea are Pelagic & neotenic, found in the sea and except polar regions.
Body is 2 mm long and surrounded by a large house and not adhere to animals.
Movement of the tail causes food and water to enter the house through apertures.

Sieve tubes form filtering apparatus.
House has an emergency exit door through which animals can escape.
Animal has an oval trunk and a laterally compressed tail attach to the ventral side.

Class – Ascidiacea

Atlantic ocean and Mediterranean sea, fixed to rocks, and 10-12 cm long.
Test thick and cartilaginous.
Body cylindrical with broad base –fixed

Free end has a branchial aperture and some distance from it on one side –atrial aperture.
Siphon is short, The pharynx is large with numerous stigmata, and the Intestine contains typhlosole.
There is no liver.

Hermaphrodite is with a single gonad in the intestinal loop.

Class – Thaliacea

This class has Free-swimming, pelagic found in cold and warm oceans.
Dimorphic – Exhibits an alternation of generations in its life cycle. Asexually reproducing solitary phase and asexually reproducing gregaria phage.

solitary phase is represented by a solitary Gonozooid about 1-2 cm long.
They are Barrel-shaped with branchial and atrial apertures situated at opposite ends.
Test of Thaliacea is gelatinous, thin, tough, transparent, and contains no cells.

The body wall contains 8 muscle annular muscle bands that completely encircles the body. The first and last bands serve as terminal sphincter muscles.
Tentacles are absent.
The pharynx is large, occupies the anterior half of the body.

Swims by forcefully ejecting water out through the atrial aperture.
Gill slits strongly arched from M2, dorsally to just past M5, then curving and running to M3 ventrally, Endostyle short, and from M2 to M4
The alimentary canal is with a short oesophagus, small stomach, and curved intestine.

The heart is posterior to endostyle in the midventral line.
Hermaphrodites gonads situated left of the middle line, ducts opening into atrial cavity.
Ovary containing several small and sub-globular ova, the testis is elongated and narrow.

References and Sources

https://www.biologyonline.com/dictionary/chordata