Phylum Hemichordata and Chordata falls under the Superphylum Deuterostome. They include those animals where the blastophore becomes the larval anus during their embryonic stages of development.
The main characteristic that Hemichordates share with Chordates is the presence of the three fundamental chordate characters in both groups viz. a notochord, central nervous system, pharyngeal gill-clefts, and a post-anal tail. Both are bilaterally symmetrical, triploblastic, and have a true body cavity and coelom.
- Characteristics That Hemichordates Share With Chordates
- 1. Both Hemichordates and Chordates are Deuterostomes
- 2. Both are triploblastic and coelomate
- 3. Both are bilaterally symmetrical
- 4. Both have organ-system level of body organization
- 5. Both have Pharyngeal gill slits
- 6. Both have post-anal tail
- 7. Chordates have true notochord & Hemichordates have false notochord
- 8. Both share common ancestor
- 9. Both have dorsal tubular nerve chord
- 10. Both have Central Nervous System
As stated by Bateson in the year 1887, Hemichordates have been included in the Phylum Chordata of the animal kingdom and there are various close relationships between the Hemichordates and Chordates.
But now it is placed as a separate phylum under non-Chordata. It’s because the notochord of hemichordate is not a true notochord but stomochord. That’s also why Hemichordates are called half Chordates.
Although there are a lot of differences between the two but, there are a few characteristics that they both share as well.
In fact, Hemichordate is a connecting link between non-chordates and chordates. It’s because it shows characteristics that are present in both the phyla thus expressing an evolutionary relationship.
Characteristics That Hemichordates Share With Chordates
1. Both Hemichordates and Chordates are Deuterostomes
Superphylum Deuterostomes includes both the Hemichordates and the Chordates phyla all on the basis of embryonic development features.
Due to this common classification group, both show a few common characters.
In both hemichordates and chordates, during embryonic development, a zygote first develops into a hollow ball of cells, called a blastula.
The early embryonic cell divisions show radial cleavage occurring parallel or perpendicular to the polar axis.
The fate of their embryonic blastomeres is of Indeterminate and Regulative development type.
One important and unique characteristic of the deuterostomes is that the Blastopore becomes the adult anus. And then, the formation of mouth takes place from a second opening on the dorsal surface of the embryo.
2. Both are triploblastic and coelomate
Triploblastic means that the developing embryo of the organism has a third germinal layer the mesoderm, in between the ectoderm and the endoderm.
Coelomate means that the body cavity that is lined by mesoderm is well present between the body wall and the gut wall.
Both the animal body factors of being Triploblastic and Coelomate are very important aspects of the Classification of organisms.
Here it is also important to note that, both Hemichordates and Chordates are Triploblastic and Coelomate in nature.
3. Both are bilaterally symmetrical
Bilaterally Symmetrical means that the animal body can be divided into identical left and right halves in only one plane passing through the body.
Here, both Hemichordates and Chordates are bilaterally symmetrical.
In Hemichordates, they have a bilaterally symmetrical body with true coelom, a tube-like gut, straight or U-shaped body with a terminal anus.
The body can be divided into three parts viz. a proboscis, a collar, and a trunk.
In Chordates, you can find bilaterally symmetry as well. Bilateral symmetry is not unique to chordates and you can see mirror images of the body if you think about the plane of symmetry that is passing through the centre of the body.
4. Both have organ-system level of body organization
In the case of both Hemichordates and Chordates, the organ system level of body organization can be seen.
The organ-systems in Chordates are highly advanced and more specific than those of the Hemichordates.
The Organ-system level of body organization means that not only do the organisms have different organs to carry out different functions but many organs combine to form a complete system and many such systems are interconnected to each other for the proper functioning of the body.
Thus in both types of organisms, you can see the digestive system, respiratory system, blood vascular system, excretory system, well developed reproductive system, central nervous system, sense organs, and various larval and developmental stages with organs, etc.
5. Both have Pharyngeal gill slits
Those openings in the pharynx region that develop into gills arches are termed as Pharyngeal Gill slits.
Recent studies indicate that the pharyngeal Gill slits that are found in Hemichordates and Chordates are homologous in a molecular sense.
In Hemichordates, the Pharyngeal Gill slits are filter-feeding organs with repeated openings along the pharynx caudal to the mouth.
The gill slits allow the movement of water in the mouth and out of the pharyngeal slits, along with it helping in the exchange of gas.
In Chordates, the Pharyngeal Gill slits are just openings between the pharynx, or throat, and the outside. It also helps in filter-feeding and the exchange of gas.
6. Both have post-anal tail
The post-anal tail is a posterior elongation of the body, extending beyond the anus. Both Hemichordates and Chordates have this feature in their body.
Hemichordates class Enteropneusta (Acorn worms) shows the presence of a post-anal tail in some species that sometimes shows weak signs of segmentation.
A post-anal tail can highly be seen in the juvenile members of the acorn worm family Harrimaniidae.
All chordates have a post-anal tail. In cases of fishes, etc. this has been highly modified into anal fins. In some species, like humans, this feature is only present during the embryonic stage.
7. Chordates have true notochord & Hemichordates have false notochord
It’s true that both Chordates and Hemichordates have notochord in common which is a supporting structure of the body.
But, there’s a little bit of difference between the notochord of the two. The notochord of Hemichordate is not a true notochord but stomochord.
Although both possesses a notochord but the structure and the composition between the two is structurally and histologically different.
In Hemichordates, the stomochord is just a flexible, hollow-tube like structure which extends dorsally from the pharynx into the proboscis and serves to communicate with the oral cavity. Thus, supporting the body.
In Chordates, the notochord is true and is like a flexible, rod-shaped structure that is found in the embryonic stage which is later replaced by the vertebral column (spine) in most adult vertebrates.
8. Both share common ancestor
Hemichordata and Chordata are the two deuterostome phyla. Chordates are known to share the same ancestral origin along with the phylum Echinodermata and Hemichrodata.
Heichordates are highly considered a connecting link between chordates and non-chordates because it shows characteristics which are present in both the phyla.
The proper phylogenetic relations between hemichordates and chordates can be seen based on the presence of the three fundamental chordate characters in both groups. These characters are the presence of a notochord, central nervous system, and pharyngeal gill-clefts.
Moreover, Hemichordates shows various affinities with organisms belonging to Phylum Chordata, Annelida, and Echinodermata.
9. Both have dorsal tubular nerve chord
A hollow dorsal nerve cord is found in all chordates, including tunicates (in the larval stage). Some hemichordates also have a tubular nerve cord.
In the early embryonic stage of the Hemichordates, this tubular nerve chord looks like the hollow nerve cord of chordates.
Thus, one thing for sure is that both Hemichordates and Chordates have a dorsal tubular nerve chord in their body in one stage of their embryonic development which can later change to another form or, it may be there throughout their life.
In Chordates, the dorsal nerve cord is a unique feature and is actually present as a hollow cord dorsal to the notochord. It is formed from a part of the ectoderm that rolls, forming the hollow tube.
In Hemichordates, they have nerve cords in the dorsal midline and ventral midline. The dorsal nerve cord is divided into the proboscis, collar cord, and trunk part of their body.
10. Both have Central Nervous System
Both Hemichordates and Chordates have a central nervous system. It was just recently discovered by Scientists that Hemichordates possess a simple centralized nervous system.
In Hemichordates, the nervous system maintains all the regulatory functions of the body. It primarily consists of two centralized nerve cords and the basiepidermal nerve net.
The two nerve cords run along the dorsal and ventral midlines. The dorsal nerve cord passing through the proboscis forms a tubular structure in the collar region and then extends into the trunk portion of the body.
In Chordates, the central nervous system extends dorsally while forming various peripheral nervous system. The central nervous system also forms the brain and the spinal cord in chordates.
The CNS in case of Chordates is also ectodermal in origin.