Protochordata Overview and Key Characteristics

Protochordate is organisms that belong to the lower chordates. Though not part of a proper taxonomic group, these organisms form a major part of Chordata. Protochordate 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.

Protochordate is 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 protochordate or only during the stages of early embryonic development. 

• Protochordate is generally found to be living in marine water. 

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

• Bodies of protochordate are triploblastic, bilaterally symmetrical in synchronization with the organ system level of organization. 

• A post-anal tail to ensure the balance is present across most protochordate.

• The pharynx in protochordate 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 protochordate is ventral to the nerve cord.

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

Classifications of Protochordata

Protochordate are an informal animal classification (i.e., not a taxonomic classification system) used to define invertebrate species that are closely related to vertebrates. The Phylum Hemichordata, as well as the Subphyla Urochordata and Cephalochordata, make up this group.

The animal kingdom's phylum Chordata is divided into two sub-groups: protochordate/acraniata (without a brain box or skull) and euchordata/Craniata (with a brain box or cranium).

Urochordata/Tunicata (notochord present only in larval tail, e.g. Salpa, doliolum) and Cephalochordata (notochord present only in larval tail, e.g. Salpa, doliolum) are the two sub-phyla of protochordate (notochord extends from head to tail region, eg. Branchiostoma).

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 fibers that are smooth in nature.

• After 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 colorless 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 fibers.

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

• Asexual reproduction is common. Other reproductive traits include internal or external fertilization.

• Examples of hemichordate 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 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 the 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 does 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 is known to live by burrowing and are free swimmers.

• An example of Cephalochordata includes Amphioxus.

What Makes Protochordate Different From Vertebrates?

Chordates are divided into two groups: 

• Protochordate

• Vertebrates

A notochord is known to exist in chordates (either at a certain stage of life or throughout the lifespan). 

Protochordate are creatures that have a notochord at some point in their lives. They are devoid of a vertebral column (i.e. an evolved notochord). They are then separated into two categories:

• Urochordate

• Cephalochrodate

During their larval stage, urochordates have a notochord. Adulthood, on the other hand, causes them to lose it (due to metamorphosis). They are classified as protochordate because they lack a vertebral column and exhibit the presence of a notochord at a certain stage of development.

Throughout their lives, cephalochordates have a notochord. As a result, protochordate is known to exist.

Vertebrates are creatures with a well-developed notochord, also known as the Vertebral Column. The point is if they do not have a notochord, why are they classified as chordates? The obvious answer is that the Vertebral Column is a fully evolved notochord. However, the more accurate response is that Vertebrates have a notochord in their bodies during the embryonic stage (as an embryo in the mother's womb), which develops into the vertebral column.