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:
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.
Protochordata are divided into three most important sub-categories based on the kind of notochord they possess, namely Hemichordata, Urochordata and Cephalochordata.
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 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 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.
Cephalochordata are known to live by burrowing and are free swimmers.