Protochordata- Characteristics, Classification, Example and More

Protochordates were the first formed chordates that are marine and are triploblastic, bilaterally symmetrical and have organ system level of organization. They also possess a true body cavity called coelom.

Characteristics of Protochordata

  • The members of Group-Protochordata (Gr. Protos = first, Chorda = cord) or, Arcania are primitive or lower chordates without a distinct head.
  • The protochordates differ mainly from the vertebrates in not having a backbone (vertebrae).
  • They possess body level organization.
  • Bilaterally symmetrical and triploblastic.
  • They have true body cavity also known as coelom.
  • Notochord is not present throughout the life cycle of all protochordate.
  • Relatively small sized animals and Cranium or skull is absent.
  • Vertebral column, jaws and brain are absent.
  • Dorsal tubular nerve cord, gill slits are usually present.
  • Sexes maybe separate or united.
  • Paired appendages are wanting.
  • They are exclusively marine forms.
  • Solitary, colonial, free-living, pelagic, burrowing or tube like living forms.

Classification of Protochodata

Classification of Protochordata
Classification of Protochordata

Subphylum 1-Hemichordata

  1. Gr., Hemi = half; Chorda = cord.
  2. The body is vermiform and is divisible into proboscis, collar and trunk.
  3. The notochord is doubtful, represented as a buccal diverticulum and is present in the anterior region of the body only.
  4. The nerve cord is absent, the nerve plexus is sub-epidermal in position.
  5. Numerous paired gill slits present or none.
  6. The blood vascular system is simple and on the pattern of annelids.
  7. Sexes separate with direct development. Indirect development includes the tornaria larva stage.

Class 1-Enteropneusta

  1. Solitary, burrowing, marine worms called tongue worm.
  2. The alimentary canal is a straight tube.
  3. Gill slits are numerous.
  4. Example-Balanoglossus.

Class 2-Pterobranchia

  1. Sedentary, colonial, marine.
  2. The proboscis bears ciliated tentacles.
  3. One pair of gill slits or none.
  4. Alimentary canal ‘U’-shaped.
  5. Example-Cephalodiscus.

Subphylum 2-Urochordata (Tunicata)

  1. Exclusively marine, sea squirts.
  2. The body is covered with a cuticular tunic or test.
  3. The notochord is present in the posterior half of the larva and absent in the adult.
  4. The dorsal tubular nerve cord is present in larva and degenerates in adult.
  5. Gill slits numerous or one pair only.
  6. The coelom is absent. An atrial cavity is present.
  7. The adult body is degenerate and unsegmented.
  8. Excretion by neural gland and pyloric gland.
  9. The development includes a free-swimming larval stage with advanced characters. Metamorphosis is retrogressive.

Class 1-Larvacea

  1. Free-swimming, pelagic forms.
  2. Neotenic, retain larval forms through life with persistent tail, notochord and nerve cord.
  3. Locomotory appendage, the tail is present.
  4. Test forming a temporary house renewed periodically.
  5. Gill slits one pair opening outside directly.
  6. Example-Oikoplura.

Class 2-Ascidiacea

  1. Fixed or free-swimming marine forms.
  2. Test is permanent and well-developed.
  3. The branchial sac is large, with numerous gill slits.
  4. Larva free swimming with highly advanced chordate characters.
  5. Adults are fixed and degenerate after the larva has undergone a retrogressive metamorphosis. The tail, the notochord is lost, the nerve cord is reduced to solid nerve ganglion.
  6. Examples-Herdmania, Ascidia.

Class 3-Thaliacea

  1. Free swimming pelagic forms.
  2. Test permanent and transparent.
  3. Circular muscular bands present in the body wall.
  4. Pharynx with two large or many gill slits.
  5. Sexes are united, the larval stage may be absent.
  6. Life history with alternation of generations.
  7. Examples-Salpa, Doliolum.

Subphylum 3-Cephalochordata

  1. Gr. Cephalas = head; Chorda = cord.
  2. Marine solitary forms generally found buried in sand in the intertidal zones of the sea coast.
  3. Small, fish-like animals, measuring up to 8 cm. in length.
  4. The notochord is persistent and runs throughout the body.
  5. The pharynx is perforated by numerous gill slits.
  6. The nerve cord is dorsal and tubular.
  7. The body shows external segmentation.
  8. The coelom is well-developed.
  9. A characteristic “Wheel organ” is present to direct microscopic food particles into the gut.
  10. Sexes separate.

Class Leptocardii

Examples: Amphioxus (Branchiostoma), Asymmetron.

Some Examples of Protochordata

BALANOLOSSUS

Classification of Balanoglossus

PhylumChordata
GroupProtochordata
SubphylumHemichordata
ClassEnteropneusta
GenusBalanoglossus

Distribution

It is cosmopolitan in distribution.

Habits and Habitat

  1. It is a tubiculous or burrowing animal generally found in shallow water of warm and temperate oceans.
  2. It lives in U-shaped burrows.

Characteristics of Balanoglossus

  1. It is commonly known as Acorn worm or Tongue worm.
  2. It is a marine, burrowing worm-like animal.
  3. The body is soft and cylindrical, having a ciliated surface.
  4. The body is divisible into a conical proboscis, ring-like collar and a long trunk.
  5. The proboscis is a thick, muscular structure enclosing the proboscis coelom, opening to the outside by proboscis pore.
  6. The collar is short, muscular and ring-like, enclosing collar cavities opening to the outside by a pair of collar pores.
  7. The trunk is superficially segmented. It is divisible into the anterior branchiogenital region, a middle hepatic region and a posterior abdominal region.
  8. Many gill slits are present in branchiogenital region, gonads are present in the genital wings.
  9. A double row of hepatic caeca is present in the hepatic region.
  10. The alimentary canal is straight, begins with the mouth and ends in the anus.
  11. Sexes are separate. Tornaria larva is present.
Balanoglossus Labelled Diagram
Diagram of Balanoglossus

HERDMANIA

Classification of Herdmania

PhylumChordata
GroupProtochordata
SubphylumUrochordate
ClassAscidiacea
GenusHerdmania

Distribution

It is widely distributed and very common in the Indian sea.

Habits and Habitat

  1. Exclusively marine and solitary, sometimes two or more animals live near to each other.
  2. It is a sedentary ascidian found fixed over the gastropod shell, but the majority are found attached to some rock or substratum by a flat foot.

Characteristics of Herdmania

  1. It is commonly known as Sea squirt.
  2. The animal is attached to the substratum at its posterior ventral end by afoot.
  3. The body is roughly oblong in outline, about 9.5 cms. to 12 cms. long and enclosed in soft leathery test.
  4. The fresh specimens are pink.
  5. The free end of the body is provided with two external openings, the branchial and atrial apertures.
  6. The mouth is present in branchial aperture and the anus is present in the atrial aperture
  7. The pharynx is sac-like, perforated by many gill slits.
  8. The alimentary canal is “U”-shaped.
  9. The blood vascular system is an open type, the heart beats show reversed peristalsis.
  10. The excretory organ is a neural gland placed over a nerve ganglion.
  11. Hermaphrodite, gonads only a single pair.
  12. Tadpole larva shows retrogressive metamorphosis.
Herdmania (Sea squirt)
Herdmania (Sea squirt) Diagram

DOLIOLUM

Classification of Doliolum

PhylumChordata
SubphylumUrochordate or Tunicate
ClassThaliacea
OrderDoliolida or Cyclomyria
GenusDoliolum

Distribution

  • Doliolum is a pelagic, free-swimming thaliacean of tropical sea waters.

Habits and Habitat

  • It is marine, pelagic, free living form.

Characteristics of Dololium

  1. Body transparent, cask-like, about 1 to 1.5 cms. with oral aperture and atrial aperture at opposite ends.
  2. The edges of the two aperture are having sensory lobes.
  3. The body is encircled by nine complete muscle bands.
  4. The oesophagus, stomach, digestive glands, intestine and the heart form a visceral mass situated after the pharynx.
  5. Nerve ganglion present.
  6. Doliolum has two phases – 1. Oozoid or asexual phase and 2. Blastozooids or sexual phase.
  7. Oozoid reproduces asexually by budding stolon.
  8. Blastozooid is protogynous, the egg is shed in the water after fertilization.
  9. The egg develops into tailed larva, the young oozoid.
Diliolum Labeled Diagram
Diliolum Labeled Diagram

SALPA

Classification of Salpa

PhylumChordata
SubphylumUrochordate
ClassThaliacea
OrderSalpida or Hemimyatra
GenusSalpa

Distribution

  • Salpa is a pelagic thaliacean of tropical sea waters.

Habits and Habitat

  • Salpa is found in open sea. They are found on surface and down upto a depth of 200 meters.

Characteristics of Salpa

  1. The body is barrel-shaped, covered by a transparent test and the size of the animal varies from 1 to 8 cm.
  2. The oral and atrial apertures are at opposite ends.
  3. The animal moves about by forceful ejection of water through the atrial aperture and active contraction of muscle bands, which are usually 6 to 7, which go round the body but are incomplete ventrally.
  4. The pharynx is large with a ventrally situated endostyle.
  5. The visceral mass consists of the oesophagus, stomach and intestine in the posteroventral part.
  6. A heart and nerve ganglion with a single, large, horseshoe-shaped sensory organ is present.
  7. Salpa has two phases: (i) the oozoid or asexual phase, and (ii) the blastozooid or sexual phase; the two alternating with each other in the life history.
  8. A stolon is formed from the ventral side, buds arise from the stolon forming a long chain of zooids; the latter detach and become free individuals.
  9. The sexual form is hermaphroditic and viviparous.
Salpa Diagram
Salpa Diagram

OIKOPLEURA

Classification of Oikopleura

PhylumChordata
SubphylumUrochordate
ClassLarvacea
GenusOikopleura

Distribution

It is a small larvacea found in almost all sea.

Habits and Habitat

  • It lives in the upper zones of the sea.

Characteristics of Oikopleura

  1. Oikopleura is enveloped into a spacious gelatinous test known as house.
  2. The house possesses an in-current and ex-current pore.
  3. The house is secreted by a special portion of the dermis known as the Oikoplastic epithelium.
  4. Water currents enter through an incurrent pore and are filtered by a filter apparatus.
  5. During filtration, the solid food particles are left in a small trough which is sucked in by the animal whose mouth lies very near to the trough.
  6. The water current also serves as hydrostatic and respiratory current.
  7. The body of the animal has an oval trunk and a long laterally compressed tail.
  8. The alimentary canal is “U“-shaped with a small endostyle and rod-like notochord.
  9. The house has an emergency back door through which Oikopleura escapes in time of danger.
  10. The animal is hermaphrodite, has gonads in the posterior region of the body.
  11. During development, it retains larval character throughout life; thus illustrates the neoteny or paedomorphism.
Oikopleura Labeled Diagram
Oikopleura Labeled Diagram

AMPHIOXUS

Classification of Amphioxus

PhylumChordata
SubphylumCephalochordata
GroupProtochordata
GenusAmphioxus

Distribution

It is widely distributed in temperate and tropical sea coasts. It is very common in Japan.

Habits and Habitat

  • It is a burrowing animal but can swim freely in shallow water.
  • It is a ciliary feeder.

Characteristics of Dololium

  1. It is commonly known as Lancelet or Lancet.
  2. Amphioxus is a small fish-like cephalochordate growing up to the length of five cm.
  3. The body is elongated, laterally compresses and pointed at both the ends.
  4. The continuous fin is divided into a dorsal, a ventral and a caudal fin.
  5. The anterior end projects forwards as rostrum.
  6. There are two metapleural folds.
  7. The myotomes arranged on both the sides of the body, separated by V-shaped connective tissue partitions called myocommata or myosepta.
  8. The mouth is ventral, guarded by oral cirri and hood.
  9. The atriopore is mid-ventral and lies at the junction of ventral fin and metapleural folds.
  10. The anus lies on the left side.
  11. The gonads are 26 pairs and are metamerically arranged on both the sides of the pharynx.
  12. The sexes are separate, sexual dimorphism is not well pronounced.
Amphioxus Diagram
Amphioxus Diagram (A) Lateral View (B) Ventral View

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