Affinities of Hemichordata
Affinities of Hemichordata (here in reference with Balanoglossus) is mainly seen with Chordata. Due to their peculiar anatomical organisation and embryology, the Hemichordata (or Balanoglossus) have been considered closer to the Chordata as well as most non-chordate phyla by different workers from time to time. Some of these views regarding the phylogenetic relationship (affinities) and taxonomic position of the hemichordates are summarized below:
Affinities with Chordata
There is close affinity between Hemichordates and Chordates. Their resemblance was based on the presence of the three fundamental chordate characteristics in Hemichordata, that is,
- a notochord,
- a dorsal hollow nerve cord and
- the pharyngeal gill-slits (pharyngotremy).
Affinities with Urochordata
Hemichordates are nearest to Urochordata, as they exhibit many close resemblances with them. The structure and function of pharynx and branchial apparatus in hemichordates are similar to those of urochordates. Also, the development of the central part of nervous system is quite similar in both.
Affinities with Cephalochordata
Besides similarities in the structure and function of the branchial apparatus, the hemichordates also show similarity with cephalochordates in the arrangement of coelomic sacs and in development.
Due to these similarities Hemichordata had been considered as a subphylum of the phylum Chordata till recently, representing its lowest group, and probably having a common ancestry.
Difference from Chordates
However, the hemichordates are no longer included under chordates because they do not possess chordate characters in a typical condition. The main objectives are:
- A true notochord does not occur in hemichordates. Unlike that of the chordates, the so-called ‘notochord’ is very short, confined to proboscis and without any supporting function. It is ventral to the main (dorsal) blood vessel and not covered by sheaths. Instead of being solid and made of vacuolated cells, it is hollow and lined by epithelial cells. It does not originate from the roof of larval archenteron but as a forward hollow projection of the foregut. Instead of being called notochord, it is now termed the Some prefer to name it as buccal diverticulum.
- The nervous system is distinctly of the invertebrate type being intra-epidermal in position and having a ventral nerve cord and a circumenteric nerve ring which area absent in chordates. In Balanoglossus, the dorsal tubular nerve cord is confined to the collar region only.
- Gill-slits of Balanoglossus are numerous and dorsal in position, whereas they are 5 to 7 and lateral in higher chordates.
- Lacking metameric segmentation, cephalization, paired appendages, post-anal tail, exoskeleton, living endoskeleton, dermis, liver, haemoglobin, red blood corpuscles, etc.
- Having peculiar division of body and coelom (into proboscis, collar and trunk), single-layered ciliated epidermis, hepatic caeca, dorsal heart, open neurocoel, colourless blood, numerous gonads, etc.
Affinities with Annelida
Hemichordata shows following affinities with Annelida which was first suggested by Spengel in 1893:
- Body is vermiform and coelomate.
- Burrowing habit, tubicolous life and ingesting mud which is passes out as castings through anus.
- Collar of Balanoglossus similar to clitellum of earthworm.
- Proboscis and prostomium similar and preoral.
- Similar arrangement of blood vessels with flowing anteriorly in dorsal vessel and posteriorly in ventral vessel.
- Dorsal position of heart.
- Tornaria larva of Balanoglossus shows several structural resemblances with the trochophore larva of Annelida in being pelagic, ciliated, with apical plate, eye spots, sensory cilia and well developed alimentary canal with similar parts.
Differences from Annelida
- Annelids do not have pharyngeal gill-slits, stomochord or buccal diverticulum and dorsal tubular nerve cord found in
- Balanoglossus does not have double and solid ventral nerve cords and nephridia found in annelids.
- In tornaria larva of Balanoglossus, preoral or proboscis coelom is present, nephridia are absent and blastopore becomes anus of the adult (Deuterostomia). In trochophore larva of annelids, preoral coelom is absent, nephridia present and the blastopore becomes the mouth (Protostomia).
Thus, compared to their differences, similarities of the two groups are only superficial and quite insignificant indicating probably a convergent evolution due to similar habits and habitat.
Affinities with Echinodermata
Affinities of Hemichordates is also seen with Echinoderms, in following characters:
Adult hemichordates and echinoderms are structurally quite different and it difficult to suspect any phylogenetic relationship between them. They show few resemblances such as:
- Entero-coelic origin of coelom and its division into three successive parts filled with sea water to serve a hydraulic mechanism.
- Heart vesicle and glomerulus of enteropneusts are considered homologous to the dorsal sac and axial gland of echinoderms. Both the structures are related and combine vascular and excretory functions.
- Nervous system is poorly developed and forms epidermal nerve plexus.
- Proteins and phosphagens present hemichordates closely resemble those echinoderms.
- Common habits and ecological niches and remarkable power of regeneration.
- Larva in both is small, pelagic, transparent and oval.
- Identical ciliated bands taking up a similar twisted course.
- Enterocoelic origin and similar development of coelom.
- Proboscis coelom opening to outside by proboscis pore of tornaria comparable to hydrocoel of echinoderm dipleurula.
- Blastopore becomes the anus (Deuterostomia) and digestive tract is complete with mouth, anus and same parts.
Differences from Echinodermata:
The protocoel is single in tornaria but paired in echinoderm larva. This raises doubt in affinity of hemichordates with echinoderms. Some believe that similarities in both, is only because of convergent evolution due to same mode of habits and habitat.
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