In this article we will discuss about the affinities and phylogeny of balanoglossus.
Affinities of Balanoglossus:
The position of Hemichordata, in the scheme of classification of animals, has been controversial. In 1814, Sedgwick and Huxley suggested the affinities of Enteropneusta (Hemichordata) with the vertebrates and it was in 1885 Bateson considered this group as a subphylum of the phylum Chordata. Metschinkoff (1865) stated that Enteropneusta had certain affinities with Echinodermata. Spengel (1893) showed the relationship of Enteropneusta with Annelida.
But on the basis of general organisation, some recent workers, such as Van der Horst (1939), Dawydoff (1948), Marcus (1958) and Hyman (1959) have thought it proper to remove this group from phylum Chordata to give it the status of an independent invertebrate phylum. The name “Hemichordata” is, however, retained for the group because it suggests that its members are related to chordates, i.e., they are “half or “part” chordates, a fact that is undisputed.
Affinities of Balanoglossus (Enteropneusta, Hemichordata) with Chordates and Non-Chordate Phyla are as follows:
1. Affinities with Chordata:
ADVERTISEMENTS:
Bateson (1887) included Hemichordata in phylum Chordata, since then a close relationship has been acknowledged between hemichordates and chordates.
Resemblances:
The phylogenetic relationship of hemichordates and chordates is based on the supposed presence of the three fundamental chordate characters in both groups, viz., a notochord, central nervous system, and pharyngeal gill-clefts. The structure and function of pharynx and branchial apparatus are similar to those of Cephalochordata and Urochordata. Origin of coelom is similar in both Hemichordata and Cephalochordata, it is enterocoelous.
The buccal diverticulum or stomochord of hemichordates has been regarded as the equivalent of a notochord since the time of Bateson.
ADVERTISEMENTS:
Modern workers of hemichordates do not accept this idea and have raised many objections:
1. The buccal diverticulum is a hollow evagination of the anterior wall of the buccal cavity, and it is not definite whether it is endodermal or ectodermal in origin, whereas the notochord is a long solid rod formed from the roof of the archenteron.
2. The buccal diverticulum is generally made of ordinary epithelial cells, while the notochord of vertebrates consists of large vacuolated cells.
3. The buccal diverticulum has no enclosing sheath as found around the notochord.
ADVERTISEMENTS:
4. The buccal diverticulum lies ventral to the dorsal blood vessel, whereas the vertebrate notochord is always dorsal to the main dorsal blood vessel.
5. The buccal diverticulum is small and confined to the proboscis, while the notochord extends far backwards. It can be safely concluded that there is no representative of the notochord in hemichordates.
There are certain resemblances between the nervous system of hemichordates and chordates, such as its position, and formation of the dorsal nerve cord from the dorsal epidermis, and the collar cord which often has a neuropore and is comparable with the brain of vertebrates.
But there are major differences, such as:
(i) Its superficial position in contact with the epidermis,
(ii) Possession of a main ventral nerve cord, and
(iii) A circumenteric nerve ring, and collar cord is solid not hollow as found in true chordates, in these features the nervous system is distinctly invertebrate.
Thus, the invertebrate features of the nervous system of hemichordates outweigh its chordate characters.
The chief link between the hemichordates and chordates lies in the pharynx and its gill- clefts. The details of the branchial apparatus having tongue bars, M-shaped skeletal rods and synapticula are exactly like those of Amphioxus. But the endostyle and epibranchial groove are absent from the pharynx of hemichordates. Such similarity can be only due to common ancestry, and phylogenetic relationship of hemichordates and chordates cannot be denied.
ADVERTISEMENTS:
Differences:
But the inclusion of hemichordates in phylum Chordata cannot be justified on the basis of a few similarities which are more than outweighed by important differences.
The main differences are:
1. Chordates do not have the body and coelomic regions corresponding to those of hemichordates.
2. The circulatory and nervous systems of hemichordates are like those of invertebrates.
3. There is no post-anal tail in hemichordates.
4. Chordates are metamerically segmented animals; this segmentation is clearly shown by the muscular, nervous, circulatory and excretory systems, whereas there is a total absence of segmentation in hemichordates.
2. Affinities with Annelida:
Spengel (1893) first suggested the relationships between hemichordata and Annelida.
Resemblances:
The main resemblances of Hemichordata with Annelida are as follows:
1. The general body form and burrowing habit of tubicolous forms such as Balanoglossus are alike and mud is ingested in burrowing. It is passed out from the anus as castings.
2. The collar of Balanoglossus resembles with the clitellum of earthworm.
3. The proboscis of Balanoglossus and prostomium of earthworm are similar and both are pre-oral.
4. The vascular system of most hemichordates is like that of annelids with blood flowing anteriorly in the dorsal vessel and posteriorly in the ventral vessels.
5. The hemichordate tornaria larva appears like a modified trochosphere larva of polychaete worms.
Differences:
The differences between two groups are so great that there can be no phylogenetic relationship between them.
The differences are as follows:
1. Pharyngeal gill-slits, buccal diverticulum and dorsal tubular nerve cord found in Balanoglossus.
2. Paired ventral nerve cord present in annelids is absent in Balanoglossus. Nephridia found in annelids are absent in Balanoglossus.
3. The larva of Hemichordata and Annelida also differ in the following ways:
(i) Nephridia present in trochophore larva are absent in tornaria larva.
(ii) Pre-oral coelom found in tornaria larva is absent in trochophore larva.
(iii) In trochosphere larva blastopore becomes the mouth, while in tornaria larva it becomes the anus.
3. Affinities with Echinodermata:
The adult hemichordate and adult echinoderm are so different that one cannot suspect any relationship between them the only anatomical similarity between them is their nervous system which in both cases consists of nerve net lying near the surface embedded in the epidermis.
But there is a strong affinity between the two phyla on embryological evidence, the method of formation of the gastrula and the coelom is very similar in the two phyla and for years the tornaria larva was considered to be the larva of an echinoderm. The tornaria larva shows a very striking resemblance with the auricularia larva and specially with bipinnaria of Asteroidea.
Resemblances:
The resemblance extends into the following details:
1. The ciliated band is identical and follows the same course in the tornaria and the auricularia and bipinnaria, though the telotroch and eye spots of the tornaria are absent in echinoderm larvae.
2. The alimentary canal has the same shape and the same divisions into foregut, stomach and intestine in hemichordate and echinoderm larvae.
3. In both groups the blastopore becomes larval anus.
4. The cleavage and gastrulation follow the same pattern in both.
5. The greatest and the most convincing resemblance lies in the method of formation and arrangement of coelomic cavities. In both the coelom is of enterocoelous origin and it divides into three antero-posterior parts, which in hemichordates are called proboscis coelom (protocoel), collar coelom (mesocoel), and trunk coelom (metacoel), while in echinoderms the three parts of the coelom are axocoel, hydrocoel, and somatocoel. Moreover, the proboscis coelom and collar coelom of hemichordates open to the exterior by pores through short hydroporic canals, as does the hydrocoel in echinoderms.
6. The heart vesicle of hemichordates is related to the proboscis coelom and is homologous with the madreporic vesicle of echinoderm larvae and both these structures are closely connected to the glomerulus of hemichordates and the axial gland of echinoderms which have a combine vascular and excretory function.
7. Proteins and phosphagens present in adult hemichordates and echinoderms are very similar.
Differences:
There are following differences between two groups:
1. Eye spot present in tornaria is absent in bipinnaria.
2. The apical plate and telotroch present in tornaria are absent in bipinnaria.
3. The protocoel is paired in echinoderms, while unpaired in tornaria larva.
The many embryonic resemblances between hemichordates and echinoderms are merely due to convergent evolution due to similar mode of habits and habitat.
The many embryonic resemblances between hemichordates and echinoderms cannot possibly be accidental or due to convergent evolution. The only infallible conclusion is that the two groups are closely related and that they arose from a common ancestor. Echinoderms have deviated greatly from the ancestral type, while hemichordates are closer to it. The common ancestor gave rise to echinoderms as a blind side branch, while the main line of evolution produced the hemichordates and chordates.
Conclusion:
The above affinities have led to the conclusion that echinoderms, hemichordates and chordates have arisen from a common ancestral stock, the dipleurula larva. Further, the echinoderms deviated greatly from the ancestral stock and formed blind branch in the main line of evolution. The main line of evolution continued to give rise to hemichordates and chordates.
It appears most reasonable to place them in the invertebrates as an independent phylum which has arisen from an ancestral stock that has given rise, on the one hand, to echinoderms and, on the other hand, to hemichordates and chordates.
Phylogeny and Systematic Position of the Balanoglossus:
Bateson (1885) placed the hemichordates in a separate subphylum under the phylum Chordata only due to their peculiar anatomical organisation. It represents the lowest group of Chordata. But they have only one character of Chordata, i.e., presence of pharyngeal gill-slits.
But the recent workers such as Van der Host (1939), Dawydoff (1948), Marcus (1958) and Hyman (1959) have thought it proper to remove the group from Phylum Chordata to give it the status of an independent invertebrate phylum. Since the group comprises only about 80 species, it is included in the category of a minor phylum.
The echinoderms and hemichordates both have arisen from a common ancestor probably the dipleurula larva. The dipleurula larva is very similar to the adult stage of tornaria and echinoderm larvae. But this idea was not accepted.
Groben (1923) established that the echinoderms are derived from pterobranchs of Hemichordata. Pterobranchs are more primitive than those of enteropneusts. They possess tentaculated arms, absent in enteropneusts, probably they are lost. The post-anal tail in juvenile enteropneusts, which is a relic of the stalk of pterobranch ancestor.
The close affinities of Hemichordata, Echinodermata and Chordata, etc., have led to the conclusion that they have common ancestor from which they have arisen independently. Probably the common ancestor was dipleurula larva of Bather. Recently Barrington (1965) expressed that Echinodermata deviated greatly from the ancestral stock and formed a blind branch. Hemichordata also did not stand on the direct line of ancestory but formed a divergent offshoot from the main line of chordate evolution.
Thus, it may be suggested that Balanoglossus and other chordates are running on the same field but not on the same track. Since the hemichordates arose from the ancestral line after the divergence of the ancient Echinodermata but before the rise of true chordates, they are often called prechordates.