Hemichordates are included in Phylum Hemichordata. These include worm-like marine animals.
Hemichordates were earlier included as a subphylum under Phylum Chordata. But now it is placed as a separate phylum under non-Chordata.
Hemichordates are called half chordates because of the absence of a true notochord in their body. The notochord of hemichordate is not a true notochord but stomochord. This stomochord is like a flexible, hollow tube found in their body part of the buccal diverticulum.
- Reasons why Hemichordates are called Half Chordates
The main difference between Hemichordata and Chordata is that Hemichordata contains an epidermal nervous system whereas, Chordata contains a central nervous system containing a dorsal, tubular nerve cord.
They are also called half chordates because they have only a few characteristics of Chordates in common. Some of the characteristics can only be seen during the embryonic developmental stages.
Reasons why Hemichordates are called Half Chordates
1. Presence of Stomochord
Stomochord is not a true notochord that supports the body. It is just a flexible, hollow-tube like structure found in hemichordates.
Stomochord is generally formed during the embryonic development stages of the hemichordates as an outward growing tissue from the roof of the embryonic gut anterior to the pharynx.
As the embryo develops into an adult, stomochord extends dorsally from the pharynx into the proboscis and serves to communicate with the oral cavity. Thus, giving support to the body of the organisms.
However, they are now recognized to not share histological composition to that of the notochord found in chordates, lacking the fibrous sheath characteristic of a notochord.
At present, scientists don’t agree that there are any histological composition of the notochord found in chordates with that of the stomochords, due to lacking of the fibrous sheath characteristic of a notochord in stomochords.
So in short if you say, the Hemichordates have the stomochord tube that contains a simple nervous system.
This tube provides the basis for placing the Hemichordates as a sister group to true Chordates but doesn’t approve it to becoming a true chordate and so, called half-chordate.
2. Huge difference between True-notochord and Notochord
The true notochord is always enclosed in a sheath that is lacking in Hemichordates.
The Notochord of Hemichordata is also termed as the tube-like Buccal diverticulum or Stomochord. This structure resembles like a rudimentary notochord present in the dorsal or backside and supporting axis for primitive vertebrates.
Moreover, if you just see the buccal diverticulum of Balanoglossus (Hemichordate) you will find that it has no relationship to the blastopore.
In Herichodates, the diverticulum is not rigid enough to serve the skeletal function. A true notochord is always a solid structure, which is impossible in the case of the Hemichordates.
And, the notochord of chordate lies above the dorsal blood vessel while that of Hemichordate lies below the central blood sinus of the proboscis.
3. No major and clear evidence of homology
Some DNA-based evolutionary studies suggest that hemichordates are more closely related to echinoderms than to true chordates.
Another very important reason to note is that the gene expression studies have failed to provide any major evidence for homology between the notochord and the stomochord, or between the notochord and any hemichordate structure.
During the gene expression experiments done on Hemichordates, it was seen that the anteroposterior maps of gene expression domains for 38 genes of chordate neural patterning are highly similar for hemichordates and chordates.
Amongst these, about 40% of the gene expression domains for 38 genes are not present in protostome (includes arthropods, annelids, and molluscs) maps.
It was also seen that the dorsoventral axis might have undergone extensive modification in the chordates which include the centralization of the nervous system, segregation of epidermis, derivation of the notochord, and an inversion of organization.
And, as seen in the majority of the cases of the gene maps it was identified that the chordate’s notochord and hemichordate stomochord are probably homoplasies.
Homoplasy is a character shared by a set of species but not present in their common ancestor thereby showing no evidence for homology.
Although various evolutionary pieces of evidence between the hemichordates and chordates are present but, when other potential notochord antecedents in hemichordates are examined it’s seen that no clear and major evidence of homolog is identified.
However, Hemichordate and chordate gill slits are probably homologs, but only at the level of simple ciliated circular or oval pores, lacking a skeleton.
4. Shows only a few chordate characteristics
Hemichordates share only a very few characteristics with that of the chordates. That’s why they are also called half-chordates.
Pharyngeal gill slits are present in hemichordates as well as in chordates. The true-notochord is present in chordates only but in hemichordates, the tubular nerve cord supporting the stomochord is a characteristic feature.
Both hemichordates and chordates show the presence of nerve cord, notochord, gill slits, and tail. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail matching to that of the hemichordates.
Hemichordates are bilaterally symmetrical and triploblastic animal and does have a true body cavity and coelom. Chordates are bilaterally symmetrical, triploblastic, coelomate with the organ-system level of body organization.
Hemichordata and Chordata are the two deuterostome phyla. Chordates are known to share the same ancestral origin along with the phylum Echinodermata and Hemichrodata.
During embryonic development, in both hemichordates and chordates, it can be seen that the blastopore (opening at the bottom of the forming gastrula) becomes the anus, and a new opening that forms later becomes the mouth of the developing embryo.
5. It is a connecting link between non-chordates and chordates
Heichordates are highly considered a connecting link between chordates and non-chordates because it shows characteristics which are present in both the phyla.
For example, Balanoglossus shows the presence of notochord and pharyngeal gill slits which is a characteristic feature of chordates. It also shows the presence of a heart that is dorsal just like non-chordates.
The proper phylogenetic connecting links between hemichordates and chordates can be seen based on the presence of the three fundamental chordate characters in both groups viz. a notochord, central nervous system, and pharyngeal gill-clefts.
The structure and functions of the pharynx and the branchial apparatus are similar to those of the Subphylum Cephalochordata and Urochordata of the Phylum Chordata.
Moreover, if you see the origin of ceolom, you will find it is enterocoelous in nature and is similar to both Hemichordata and Cephalocghordata.
The Hemichordates also show connecting links with the invertebrate (non-chordate) Phylum Annelida because of their burrowing habitat, collar resembling the same as clitellum in Earthworms, proboscis also resembles same like prostomium of earthworm, matching blood vascular system in both types, etc.
Hemichordates also show connecting links with the invertebrate (non-chordate) Phylum Echinodermata. Just like, the anatomical similarity between the nervous system of both the organisms consists of nerve net lying near the surface embedded in the epidermis layer.
Moreover, the formation of the gastrula and coelom is very similar in both the Hemichordata and the Echinodermata. And, in both groups, the blastophore becomes the larval anus during embryonic development.
6. These are actually worm-like marine animals
Hemichordata is a phylum of marine deuterostome animals which includes worm-like marine animals showing close relationships with that of the invertebrates.
These worm-like marine animals do not show any cranium nerves, jaws, vertebral columns, paired appendages, etc. Their body is divided into proboscis, collar, and trunk where the proboscis and collar resemble that of the invertebrates earthworms.
The Class Enterneustaof Hemichordata shows a large body with worm-like appearance having a straight alimentary canal. These are also called Acorn worms.
The front end of these animals (Acorn worms) is shaped like an acorn, hence their common name. They consist of a muscular proboscis and a collar that may be used to burrow into soft sand or mud and stay there.
This is the same as the way of living of the worm-like higher invertebrates.
Moreover, if you see, the Hemichordates’s shape of the body, structure of Proboscis and prostomium, burrowing and feeding habits, ventral nerve cord, dorsal and ventral blood vessel system, direction of flow of blood, resemblence of larvae, etc.
All these have close association with the worm-like annelids mostly resembling earthworms, ragworms, leeches, etc.