Phylogeny and affinities of balanoglossus
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This document discusses the phylogeny and affinities of Balanoglossus, which is a member of the phylum Hemichordata. It examines Hemichordata's relationships to Chordata, Annelida, and Echinodermata based on morphological and anatomical similarities and differences. While Hemichordata shares some key characteristics with Chordata, such as the presence of a notochord, dorsal nerve cord, and pharyngeal slits, it also displays important differences. Comparisons are also made between Hemichordata and Annelida based on their segmented worm-like bodies, and between Hemichordata and Echinodermata primarily through similarities seen in their lar
Phylogeny and affinities of balanoglossus
- 2. INTRODUCTION: The place of Hemi-Chordata in the Animal kingdom has been in discussion since longback. The group Enteroneusta was first established by Gegenbaur in 1870. Metscnikoff showed its affinities with Echinodermata in 1869 Spangel in 1893 showed its relations with Annelids. In 1814 itself Sedgwick and Huxley suggested affinities of Enteroneusta with Vertebrates. Bateson in 1886 argued the Chordata affinities vigourously. Finally it was Hyman (1959) who gave it a rank of independent phylum in Invertebrates
- 3. Affinities of Hemi-Chordata 1.Affinities with Chordates: Hemi-Chordates resemble Chordates in having the fundamental characters ,i.e Presence of notochord, dorsal tubular nerve chord and pharyngeal gill slits. A) Notochord: In hemichordates notochord is present in the form of buccal diverticulum which is made up of vacuolated cells as in chordates B) Nerve cord: Dorsal nerve cord of hemichordates resembles the dorsal nerve cord of chordates because it is also formed from Ectoderm. C) Gill slits: Formation of gill slits in chordates and hemichordates is in the same plan.(pharyngotremy) D) Coelom: Coelom is enterocoel in both
- 4. Differences with Chordates: A) The coelomic regions of hemichordata are not seen in Chordata. B) Post anal tail,cephalisation and paired appendages are absent in hemichordates. C) Metameric segmentation of muscular, nervous,circulatory and excretory systems are not represented in hemichordates. D) Gill slits are lateral in chordates but dorsal in hemichordates E) Buccal diverticulum is not considered as notochord as the enclosing sheath is absent and is confined to proboscis only. F) Dorsal nerve cord is confined to collar region and at other places only intra epidermal nerve strand is present. Ventral nerve cord and circumcentric ring are also present in hemichordata but absent in chordata.
- 5. Affinities with Annelida: Similarities: i.Body is segmented and worm like as annelids. ii.Resembles polycheates in borrowing (tubicolous) iii.Intra epidermal nervous system is present iv.Blood flows from back to front in dorsal blood vessel and from front to back in ventral blood vessel. v.Tornaria larva of hemichordata has many resemblences with Trochophore of Annelids.
- 6. Dissimilarities: i.Gill slits are absent in annelids ii.Paired nerve cords are present in annelids. Dorsal nerve cord seen in hemichordates is absent in annelids iii.Coelom in annelids is schizocoelous and in hemichordates is enterocoelous. iv. Cleavage of zygote in annelids is spiral and in hemichordates is radial v. Two differ in the following a) nephridia in tornaria are absent. b) pre-oral coelom is absent in trochophore c) blastopore in trochophore becomes mouth(protostome) and in tornaria it becomes anus (deuterostome)
- 7. Affinities with Echinoderms: Morphologically and anatomically hemichordates and echinoderms do not show any resemblences. The larvae of both echinoderms and hemi chordates show their affinities
- 8. Similarities: Blastopore becomes anus in both groups Gut is regionated into oesophagus, stomach and intestine. Protocoel and hydrocoel lie on the left side. The ciliated band of Tornaria resembles that of Auricularia and Bipinnaria in the position and arrangement. Regeneration power is high in both the groups. Origin of coelom is same in both the groups and it is enterocoelus with potocoel,mesoceol and metacoel. In both nervous system is poorly developed and is present below the epidermis in both the groups in the form of nerve plexus.
- 9. Dissimilarities with Echinodermates: The eye spot present in hemichordate larva is absent in Echinodermate larvae. Apical plate and telotroch are absent in Bipinnaria and Auricularia but present in Tornaria. Protocoel is paired in Echinoderm larva and unpaired in Tornaria. Conclusion: As the differences between the two groups are less , the two groups are more closely related as proposed by Hyman and is accepted worldwide.