Protochordata

Protochordata are organisms that belong to the lower chordates. Though not part of a proper taxonomic group, these organisms form a major part of Chordata. Protochordata have a notochord (a structure that provides support to the body of an organism) for a certain period during their lifetime or all throughout.  Often also known as Acraniata, these organisms do not have a proper skull and a cranium.


Protochordata are classified into three kinds of sub-phyla based on the type of notochord they possess. They are:

  • Hemichordata 

  • Urochordata

  • Cephalochordata

Characteristics of Protochordata

Here are some of the most commonly identified characteristics of Protochordata.

  • A notochord, which is a set of vacuolated turgid cells resembling a solid, elasticated and, unsheathed structure like a rod, is present throughout the lives of protochordata or only during the stages of early embryonic development. 

  • Protochordata are generally found to be living in marine water. 

  • The Central Nervous System or CNS of protochordata is single, hollow, and dorsal, which is completely unlike the CNS of non-Chordata organisms.  

  • Bodies of protochordata are triploblastic, bilaterally symmetrical in synchronisation with organ system level of organisation. 

  • A post-anal tail to ensure balance, is present across most protochordata.

  • The pharynx in protochordata is perforated by gill slits to facilitate water circulation for respiration. 

  • The heart of these organisms is ventral with a closed circulatory system.

  • The gut in protochordata is ventral to the nerve cord.

  • Examples of protochordata include Amphioxus, Salpa, Doliolum, and Saccoglossus. 

Classifications of Protochordata

Protochordata are divided into three most important sub-categories based on the kind of notochord they possess, namely Hemichordata, Urochordata and Cephalochordata.

Hemichordata 

  • All hemichordata are marine organisms. While some live in solitary confinement, others can be colonial.

  • Their bodies are unsegmented, cylindrical and stout often lending a resemblance like a worm.

  • They are triploblastic and bilaterally symmetrical.

  • The body wall has a single layer epidermis and longitudinal fibres that are smooth in nature.

  • Ofter the collar of hemichordata may have tentacles or arms, however, locomotory appendages are missing.

  • Their circulatory systems include a dorsal heart with a longitudinal dorsal and ventral vessel.

  • The blood of hemichordata is colourless and has no corpuscles.

  • They possess a complete digestive tract.

  • All hemichordata use a ciliary mechanism or use filtering to feed on microorganisms and other debris.

  • The body of hemichordata has a general surface that facilitates breathing Otherwise, respiration occurs through a pair or multiple pairs of gill slits.

  • The excretory system is made of the glomerulus or proboscis gland.

  • The nervous system of hemichordata consists of an epidermal plexus or nerve cells along with nerve fibres.

  • Sexes of hemichordata may be united or seperate, and gonads may exist in multiple pairs or one.

  • Asexual reproduction is common. Other reproductory traits include internal or external fertilisation.

  • Examples of hemichordata include Cephalodiscus, Balanoglossus, Rhabdopeura.

Urochordata

  • Urochordata are marine organisms, filter-feeders and are mostly sessile in nature.

  • The body or Urochordata is enclosed in a leathery sheath that is also called tunicates since it is composed of tunicin.

  • The notochord that only appears in the larva tail of uro chords disappears in adults due to retrogressive metamorphosis.

  • There is no presence of an excretory system.

  • The blood of Urochordates comprises of venadocytes.

  • Their circulation system is of the open kind.

  • They reproduce asexually by the process of budding.

  • Urochordata breathes through gill slits or stigma, which are numerous in number. Instead of opening into the exterior, they open into the atrium. 

  • The nerve cord that is also known as neural tube in the larva is replaced by one dorsal ganglion in adults.

  • Examples of Urochordata include Salpa, Herdmania, Doliolum

Cephalochordata

  • Cephalochordata are also marine organisms and filter-feeders

  • The tails of Cephalochordata stay throughout their lives.

  • Their body wall is made up of myotomes often lending a fish-like resemblance.

  • Cephalochordata do not have the formation of a brain instead of having the nerve cord for all their lives.

  • Their notochord persists throughout their lives, extending up to the head or the cephalic region.

  • Excretion in Cephalochordata occurs through paired protonephridia with solenocytes (a type of flagellated cells).

  • They have plenty of gill stilts which also persist while they are in the adult stage. These open into the atrium.

  • Cephalochordata are known to live by burrowing and are free swimmers.

  • An example of Cephalochordata includes Amphioxus.