J.F. Eschscholtz who discovered Balanoglossus in 1825 in Mashail Island described it as a worm-like holothurian. The discovery of gill-slits in this animal by Kowalewsky (1865) led to creation of a special class Enteropneusta by Gegenbaur (1870). Bateson (1885) included them in Phylum Chordata. Hyman (1959), however, placed them near Echinodermata and gave Hemichordata a status of an independent phylum.
This simple organism shows affinities with several diverse groups.
- Affinities with Nemertinea (Proboscis worms)
Nemertines resemble flatworms and possess a long protrusible proboscis. Balanoglossus shows similarity with nemertine in burrowing and feeding habits. The proboscis of nemertines is compared with proboscis of Balanoglossus. These similarities are superficial as the proboscis of Balanoglossus.is not protrusible as in nemertines. The Balanoglossus. also does not have lateral nerve cord of nemertines.
2. Affinities with Phoronida
These are worm-like tube-dwelling animals living in the bottom of shallow seas.
The paired sacs given off from the stomach of phoronids are compared with buccal diverticulum of Balanoglossus.
The nervous system in both the groups is intraepidermal.
The power of regeneration is seen in both the groups.
The actinotroch larva of Phoronis is similar to tornaria larva of Balanoglossus.
In spite of these similarities following differences present between these two groups show that these groups are not closely related:
1) Absence of gill-slits in Phoronis.
2) Presence of lateral nerve cord in Phoronis.
3) Formation of coelom by splitting of the mesoderm in Phoronis (schizocoelic).
The study of development of Phoronis by Selys-long Champ (1909) convincingly proved that Balanoglossus cannot be placed with Phoronida.
3. Affinities with Pogonophora (Beard worms)
These are marine tube-dwelling animals having anteriorly one or more tentacular arms.
The most striking similarity of this group with Balanoglossus is its method of formation of mesoderm and coelom. The mesoderm is formed from endoderm in the form of hollow pouches which come to lie between ectoderm and endoderm, The cavities of the pouches form the coelom. Such animals are called enterocoelomate since the mesoderm and coelom arise from the gut.
The other characters in which beard worms resemble Balanoglossus are:
1) Presence of protocoel with external pore.
2) Location of gonads in the trunk.
3) Intraepidermal nervous system.
4) Transformation of coelomic epithelium into muscle fibres and connective tissue.
They differ in the following characters:
1) The central nervous system is located in protosome of Pogonophora and in mcsosome in Balanoglossus.
2) The tentacular apparatus which is present in some hemichordates is of mgsosomal (collar) oriain while that of Pogono-phora is of protosomal origin.
The fact that only a few invertebrate groups are enterocoelomate suggests that Pogonophora appears to be fairly closely related to hemichordates.
4. Affinities with Annelida
Spengel (1893) suggested affinities of Balanoglossus with annelids.
The two groups show following similarities:
1) Shape of the body.
2) Proboscis of Balanoglossus resembles prostomium of earthworm.
3) Burrowing and feeding habits.
4) Ventral nerve cord.
5) Dorsal and ventral blood vessel.
6) Direction of flow of blood.
7) Resemblance between trochophore larva of annelida and tornaria larva of Balanoglossus in the following characters:
a) Apical plate with eyes, sensory cilia and attached muscles.
b) Differentiation of alimentary canal into oesophagus, stomach and intestine.
c) Two anterior pairs of mesodcrmic vesicles.
d) The preanal ciliated ring of trochophore corresponds with principal ciliated ring of tornaria.
The two groups differ in the following characters:
1) The dorsal nerve cord and gill-slits are not present in annelids.
2) The metamerically arranged nephridia of annelids are not present in Balanoglossus.
3) The cleavage of egg of annelids is spiral and determinate while that of Balanoglossus is radial and indeterminate.
4) The gastrulation in annelids occurs by epiboly and in Balanoglossus by emboly.
5) The coelom of annelid is schizocoelic while that of Balanoglossus is enterocoelic.
6) The blastopore becomes mouth in annelid larvae, and anus in development of Balanoglossus.
7) A pair of primitive kidneys of trochophore are absent in tornaria.
8) The pre-oral coelom of tornaria is absent in trochophore. These differences between the two groups suggest no definite relationship between them. These two groups of animals might have originated from a remote ancestral stock. The annelidan aflffmities of Balanoglossus have, therefore, been given up.
5. Affinities with Echinodermata
The affinities of Balanoglossus with echinoderms were first suggested by Metschinkoff (1865).
There are no similarities between adults of echinoderms and Balanoglossus except in the presence of intraepidermal nervous system.
However, the striking resemblance between the larval forms of the two groups misled Muller to regard tornaria larva as a larva of starfish.
The similarities between tornaria larva and bipinnaria and other larval forms of echinoderms are as follows:
1) They are pelagic and transparent.
2) The early development follows the same pattern.
3) The blastopore becomes anus.
4) The coelom is enterocoelic as in pogonophores.
5) The coelom when first formed is divided into three antero-posterior parts, the protocoel, mesocoel and metacoel in Balanoglossus and axocoel, hydrocoel and somatocoel in echinoderms. The hydrocoel opens to exterior by pore as the protocoel and mesocoel does in Balanoglossus. The proboscis pore of Balanoglossus is compared with waterpore.
6) The heart vesicle of Balanoglossus is compared with madreporic vesicle of echinoderm larvae. These structures are closely related with excretory structures, the glomerulus in Balanoglossus and axial gland in echinoderms.
7) The alimentary canal is divided into three parts, oesophagus, stomach and intestine.
8) The resemblance in ciliated bands of these larval forms is regarded as a superficial character.
The larval forms show the following dissimilarities:
1) The absence of apical plate with eye spots, cilia in echinoderm larvae.
2) The protocoel is paired in echinoderm larvae and unpaired in tornaria.
The likeness between the larval form of these two groups suggests strong affinity between them and their origin from a common ancestor. They must have evolved along different lines in response to different modes of life they were called upon to lead.
Affinities with Chordata
Bateson (1885) was the first to advocate chordate aflinities of Enteropneusta and include them into Phylum Chordata. They were supposed to possess the notochord, the gill-slits, and the dorsal tubular nerve cord which are three unique characters of the chordates.
(1) Notochord: Bateson called the buccal diverticulum of Balanoglossus as notochord for the following reasons:
(a) It is derived from the dorsal region of the endoderm.
(b) It has some relation to the skeletal function.
(c) It is made up of vacuolated cells.
This structure, as predicted by Newman, has ceased to be regarded as notochord due to the following reasons:
(1) The notochord is never formed as a diverticulum. It is cut off from the roof of the archenteron because of the formation of coelomic pouches on either side of it. The buccal cavity is usually lined with ectoderm. So it is doubtful if buccal diverticulum is an endodermal structure. The true notochord is always a solid structure.
(2) The notochord of chordates extends backwards from the head and not forwards only. It is argued that the notochord extending throughout the length of the body will be disadvantageous to the animal whose movements depend upon the lengthening and shortening of the body.
(3) The buccal cavity is lined by vacuolated cells which are continued in its diverticulum.
According to Newell (1952) the ‘notochord’ of Balanoglossus. differs from that of the true chordates in the following characters:
1. The true notochord is enclosed in a sheath which is lacking in Balanoglossus.
2. The buccal diverticulum of Balanoglossus has no relationship to blastopore.
3. The diverticulum is not rigid enough to serve the skeletal function.
4. The notochord of chordate lies above the dorsal blood vessel while that of
Balanoglossus. lies below the central blood sinus ofthe proboscis.
5. The presence of cavities in it.
Silen (1957) calls buccal diverticulum as some sort of pre-oral gut while acccording to Ganguli and Mukherji it is nervous in nature.
(2) Gill-slit. The closest relationship between Balanoglossus and the true chordates exists due to presence in it of a number of pairs of gill-slits. There is close similarity in gill-slits of Balanoglossus and Amphioxus. The primary gill-slits of Amphioxus is divided into two slits by a tongue bar which grows down from the dorsal side. The tongue bar of Balanoglossus, however, does not completely divide the slit. The skeletal rods supporting the gill bars are also more or less similar in these two animals. The urochordates which
are more closely related to Amphioxus do not have this type of gill-slits. Some authors doubt whether the gill-slits of Balanoglossus really are pharyngeal clefts as in one species of Balanoglossus, there are as many as 700 pairs of gill-slits. In higher chordates, the pharyngeal region is very short with generally 5 or 6 pairs of gill-slits.
(3) Dorsal tubular nerve cord. The portion of the dorsal nerve cord in the collar region of Balanoglossus is the dorsal tubular nerve cord. The anterior and posterior openings of this collar cord are called the anterior and posterior neuropores respectively.
Except for this region the rest of the nervous system of Balanoglossus is distinctly of invertebrate type. The ventral nerve cord of Balanoglossus never exists in chordates.
Thus except for gill-slits the alleged chordate characters of Balanoglossus seem to be decidedly questionable.
The five coelomic cavities of Balanoglossus constituting the trisegmental condition are also found in an embryonic stage of Amphioxus. The enterocoelic origin of coelom in Balanoglossus and Amphioxus is also of greater phylogenetic significance than the resemblances in the gill-slits, and so-called notochord and dorsal tubular nerve cord.
The chordates are metamerically segmented animals while Balanoglossus retains in adult stage, the trisegmental character of their larvae. According to Newman, Balanoglossus thus retains throughout life a larval organisation while Amphioxus and vertebrates have secondarily introduced metamerism by division of posterior coelom into a long scries of segments.
The epidermis of hemichordate is ciliated whereas it is nonciliated in chordates.
The direction of the flow of blood in dorsal and ventral blood vessel is similar to that of invertebrate like earthworm.
The resemblances between Balanoglossus and true chordates in some adult structures and embryonic and larval stages show that the hemichordates and the true chordates are, without question, related, however distantly. They are, as mentioned above, closer to invertebrate phyla Echinodermata and Phoronida in their morphology and development than to the Chordates.
Hence Hyman has placed them in the invertebrates as an independent phylum.