Hemichordata status Introduction
Hemichordata was till recently treated as the sub phylum of the phylum Chordate but is now regarded to be the independent phylum of invertebrates very close to the phylum Echinodermata. The peculiar characteristics of the animals belonging to Hemichordata are as follows,
- Enterocoelus coelom
- Pharyngeal gill slits
- Buccal diverticulum (earlier considered as notochord)
- Vermiform body divisible into three regions – proboscis, collar, trunk
The phylum hemichordate includes a small group of soft, primitive and marine chordates called as acron worms or tongue worms. Most familiar genus of this phylum is Balanoglossus which belongs to the class Enteropneusta. Other closely related genera are Saccoglossus, Glossobalanus etc.
The class Enteropneusta was established by Gegenbaur in 1870. Since then due to the peculiar anatomical organisation and embryology, Hemichordata have been considered closer to chordate as well as non-chordate phyla by different workers from time to time. The phylogenetic relationship and the taxonomic position of hemichordate are given below:
Affinities of the phylum Hemichordata
Affinities with Chordata
Zoologists proposed closer affinities between the phylum Chordata and Hemichordata. Their resemblance was based on the presence of three fundamental characteristics of Chordates in hemichordates,
- Dorsal hollow nerve chord
- Pharyngeal gill slits
Moreover the structure and functions of pharyngeal and branchial apparatus of Hemichordates is similar to that of Cephalochordates and Urochordates. Also the origin of coelom is enterocoelic type in the form of five pouches from the archenteron as in Brachiostoma. Due to all the above stated similarities Hemichordata was considered as sub-phylum of the phylum Chordata till recently.
Following are the objections which lead the establishment of Hemichordata as a separate phylum.
1. A true notochord does not exist in hemichordata. Unlike the notochord of Chordates, the so called notochord of hemichordates is very short, confined to proboscis and without any supporting function. Instead of being solid and made up of vacuolated cells, it is hollow and made up of epithelial cells. Instead of being called notochord, it is now referred to as Stomochord by Bateson and as buccal diverticulum by Hyman.
2. The nervous system is distinctly of invertebrate type being intra-epidermal in position and having a ventral nerve cord and circumcentric nerve ring which are absent in chordates.
3. Gill slits in hemichordates are numerous and dorsal in position where as they are 5 to 7 and laterally positioned in chordates.
Affinities with Rhynchocephalia
Some workers linked Hemichordata with Rhynchocephalia as the feeding and borrowing habits are similar in both. Body in both is elongated, vermiform, without external metamerism, with terminal anus, with smooth skin and metamerically arranged simple gonads.
- But Rhynchocephalis differ in lacking a dorsal nerve chord and having lateral nerve chord.
Affinities with Phoronida
Some Zoologists advocated relation between hemichordata and Phoronida due to similar nature of epidermal nervous system paired gastric diverticula. The larval forms of both so some similarities and both have great power of regeneration.
- But Phoronida differ in lacking Pharyngeal gill slits. More over Phoronida had paired metanephridia which are absent in hemichordata.
Affinities with Pogonophora
Some Zoologists tried to relate hemichordata with Pogonophora due to similarities in the formation of enterocoelous coelom, division of body and coelom into three regions, intra epidermal nervous system and gonads.
- But Pogonophores differ in having protocoelic nephridial coelomoducts and lacking alimentary canal. Moreover the nervous system is concentrated in protostome in pogonophores whereas in hemichordates it is concentrated in mesosome.
Affinities with Annelida
Some zoologists drew relationship between hemichordata and Annelida due to similarities in vermiform body and coelom, burrowing habits, ingesting mud which is passes out as castings through anus, arrangement of blood vessels with blood flowing anteriorly in dorsal vessel and posteriorly in ventral vessel, dorsal position of heart.
- But annelids differ in lacking pharyngeal gill slits, stomochord and dorsal tubular nerve chord. Hemichordates do not have double ventral nerve chord and nephridia as in annelids.
Affinities with Echinodermata
The following are the resemblances of the adult hemichordates and echinoderms:
- Echinoderms and hemichordates are similar in having enterocoelus coelom which is divided into three different parts filled with sea water to serve hydraulic mechanism.
- Hearth vesicle and glomerulus of hemichordates are considered homologous to dorsal sac and axial gland of echinoderms.
- Both have common habits, ecological niches and remarkable power of regeneration.
The following are the resemblances of the larval hemichordates and echinoderms.
- Larvae of both groups are small, pelagic, transparent and oval
- Similarity in the development of the coelom
- Blastopore becomes anus and digestive tract is complete with mouth, anus and same parts.
- But Echinoderm larvae differ in lacking apical plate with sensory hair, eye spots and telotroch. Protocoel is single in hemichordate larva and paired in echinoderm larva.
Systemic position of hemichordata
Peculiar anatomical organisation of hemichordata makes their phylogenetic position uncertain and controversial. Earlier workers placed hemichordates as a sub phylum under the phylum Chordata. But the only chordate feature shown by them is the presence of pharyngeal gill slits. Therefore, some recent workers like Van der Host, Marcus etc. have chosen to remove them as the sub phylum of phylum Chordata and treat them as an independent invertebrate phylum.
Since the hemichordate group consists of only 80 species, it is included in the category of minor phylum. The name Hemichordata suggests that they are half chordates.
The close affinities of Hemichordata with Echinodermata, Annelida, Pogonophora, Phoronida have led to the conclusion that they have arisen from a common ancestral stock. But many workers like Marcus, Berril and Hyman do not contribute to this view.
According to Barrington, Echinodermata deviated greatly from the line of ancestral stock and formed a blind branch. Even hemichordata did not stand on the direct main line of chordate evolution. Hemichordates arose from the ancestral line after divergence of the ancient echinoderms but before the rise of true chordates and hence they are often called as pre-chordates.
- What are the peculiar characteristics of the animals belonging to the Hemichordata?
- Discuss the affinities of Hemichordata with Chordata and Phoronida.
- Discuss the objections in relation to the affinities of hemichordata with Rhynchocephalia and echinodermata.
- Describe the systematic position of Hemichordata.
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