Different affinities and systematic position of balanoglossus are given below:
(1) Affinities with chordate:
Earlier Zoologists like William Bateson (1885) proposed close affinities with chordates on the basis of three fundamental characters:
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(a) A notochord
(b) A dorsal hollow nerve cord and
(c) Pharyngeal gill slits
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On the basis of these similarities hemichordates have been considered as a subphylum of chordata. But this view has faced the following objections-
(a) Notochord of hemichordates is not homologous to the true notochord. In fact it is a hollow projection of the fore gut and is preferably called buccal diverticulum.
(b) Nervous system is of primitive type, which resembles with invertebrates.
(c) Gill slits are numerous and on dorsal side of Balanoglossus, which do not resemble with chordates.
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(2) Affinities with rhynchocephalia :
Balanoglossus resembles with Nemertines in the following characters-
(a) Elongated vermiform body with terminal anus and smooth skin.
(b) Unicellular glands and ectodermal nerve plexus.
(c) Feeding and burrowing habits.
But Nemertines differ, in lacking a dorsal nerve cord and a protrusible proboscis.
(3) Affinities with phoronida :
Balanoglossus bears resemblances, with Phoronis in the following characters-
(a) Similar nature of epidermal nervous system.
(b) Gastric diverticula of Phoronis resembles with buccal diverticulum of Balanoglossus.
(c) Simillarities in ciliary band, hydropore, sensory organs and eye spots between the actinotorch larva of Phoronis and tornaria larva of Balanoglossus.
(d) Both have great power of regeneration
Objections:
The characters which differ from each other are the following.
(a) Pharyngeal gill slits of Balanoglossus are absent in phoronis.
(b) Paired nephridia of Phoronis are absent in Balanoglossus.
On the basis of other developmental differences the relationship of these two groups has been rejected.
(4) Affinities with pogonophora :
Marcus (1958) tried to relate Hemichordata with Pogonophora due to the following similarities.
(a) Enterocoelic coelom.
(b) Body and coelom divided into three regions.
(c) Mesosome and metasome are separated by a septum.
(d) Nervous system subepidermal.
(e) Gonads are found in trunk.
Objections:
(a) Protocoelic nephridial coelomoducts and
(b) Lacking of an alimentry canal in Pogonophora rejects any close relationship between the two groups.
(5) Affinities with annelida :
Spengel (1893) suggests the following affinities
(a) Body vermiform.
(b) Tubiculous, burrowing and ingesting mud which is passed out as castings.
(c) Similarities between collar of Balanoglossus and clitellum of annelids.
(d) Proboscis and prostomium are similar and preoral in position.
(e) Similarity in blood vascular system.
(f) Dorsal position of heart.
(g) Similarities between tornaria and trochopore larva .
Objections:
They differ fundamentally in the following characters-
(a) Pharyngeal gill slits and buccal diverticulum of Balanoglossus are absent in annelids
(b) Ventral nerve cord and nephridia of annelids are absent in Balanoglossus.
(c) Development of trochophore and tornaria show great difference in many characters.
Thus the relationship between both the groups has been rejected.
(6) Affinities with echinodermata :
Balanoglossus show following similarities with Echinoderms.
(a) Coelom is enterocoelic and divides into three cavities.
(b) Heart vesicle and glomerulus of enteropneusts are homologous to dorsal sac and axial gland of echinoderms.
(c) Nervous system is primitive in both the groups.
(d) Both groups are similar in proteins and phosphogens.
(e) Both the groups show great power of regeneration.
(f) Larva of both the groups bear identical ciliated bands and their twisted movement.
(e) Presence of protocoel and hydropore in tornaria resemble with hydrocoel of dipleurula larva.
Objections:
The presence of apical plate with sensory hairs, eye spots and telotorch in the tornaria larva makes difference with echinoderm larva. Protocoel is single in tornaria but is paired in echinoderm larva.
(7) Systematic position and phylogeny:
The systematic position of Balanoglossus is conspicuous and controversial due to some peculiar anatomical characters.
Earlier Hemichordata has been considered as a subphylum of phylum Chordata which includes primitive chordates.
But in reality, they bear only a single chordate character, the pharyngeal gill slits. In other characters they are rather more similar with invertebrates.
Therefore, some recent workers like Vander Host (1939) Dowydoff (1948), Marcus (1958) and Hymen (1959) have chosen to remove hemichordates from the phylum chordata and treat them as an independent invertebrate phylum.
Regarding phylogeny, of this group the close affinity with Echinodermata, Phoronida, Pogonophora, and chordata, has led to the conclusion that the Hemichordates have arisen from a common ancestral stock.