ADVERTISEMENTS:
In this article we will discuss about the meaning and classification of phylum hemichordata.
Meaning of Phylum Hemichordata:
The animals with notochord constitute the Phylum Chordata. It has a major subdivision, called the Vertebrata or Craniata for possession of vertebral column or cranium. The rest of chordates, namely Hemichordata, Urochordata and Cephalochordata are collectively called the Primitive Chordates or Protochordates.
But it is better to divide it into Invertebrate Chordates for Hemichordata, and Protochordata or lower chordates for Urochordata and Cephalochordata for the structural organisation of chordate features. The basic chordate features are present in Protochordata.
ADVERTISEMENTS:
Hemichordata has long been considered as the lowest group of chordates for the construction of notochord, nerve cord and pharyngeal gill-slits, the main features of the Phylum Chordata. Recent workers claim that the notochord of hemichordates is not a true notochord, and the central nervous system containing a longitudinal ventral nerve strand shows the characteristic of major invertebrate phyla.
The pharyngeal gill-slits or pharyngotremy is the only chief link between hemichordates and chordates. So the inclusion of Hemichordata in the Phylum Chordata remains a controversial issue and still remains of uncertain status (incerte sedis). Hemichordates are somewhat close to invertebrates in the body construction. So the term ‘Invertebrate Chordates’ for Hemichordata by some zoologists is partially justifiable.
The so-called protochordata including Hemichordata, Urochordata and Cephalochordata lead sedentary life in adult stage and perform ciliary mode of feeding to compensate the purpose of locomotion (Fig. 2.1).
ADVERTISEMENTS:
Phylum Hemichordata:
The phylum Hemichordata represents a group of lowest chordates having profound phylogenetic significance. The hemichordates furnish a clue to the link between the chordates in general and the non-chordates, specially the echinoderms in particular. The hemichordates have peculiar anatomical organisation and the free-swimming larva, tornaria, is strikingly similar to the larval stages of some echinoderms.
Classification of Phylum Hemichordata:
Historical Resume:
The first member of the Hemichordata was recorded by Eschscholtz (1825). He named the animal Ptychodera and regarded it as a holothurian. Subsequently, Delle Chiaje (1829) added another member Balanoglossus clavigerus to the group.
Since then, many hemichordates have been put on record. All these hemichordates were included under the group Enteropneusti (Gr. enteron = intestine and pneumon lung) as proposed by Gegenbaur (1870).
Since then, this group was called Enteropneusta. Bateson (1885) studied the embryology of some enteropneusts and proposed the name Hemichordata to replace Enteropneusta. The tornaria larva was regarded as an echinoderm larva by Johannes Miller (1850), Krohn (1854), Agassiz (1864) and many others. Metchnikoff (1870) regarded tornaria as the larval stage of Enteropneusta.
The work of Kowalevsky needs special mentioning, because he gave the accurate description of Balanoglossus. The hemichordate possessing an exceptionally elongated proboscis was named Saccoglossus by Schimkewitsch (1892), but Spengel (1893), in his monograph, treated the same animal as subgenus Dolichoglossus under the genus Balanoglossus.
Van der Horst (1939) regarded Saccoglossus and Dolichoglossus as the synonyms of the same genus.
Sars (1867) and Allman (1869) recorded the strange colonial Rhabdopleura as a member of Bryozoa. Lankester (1877) introduced the term Pterobranchia for Rhabdopleura, but still retained it as a subdivision under Bryozoa.
ADVERTISEMENTS:
Another similar animal, Cephalodiscus, was recognised by Mcintosh (1882) which was also regarded as a bryozoan. Hamer (1887) and Fowler (1892) removed Cephalodiscus and Rhabdopleura, respectively from the group Bryozoa and suggested their inclusion under Hemichordata. Another genus, Atubaria, was added to the group by Sato in 1936.
Willy (1899) divided the Hemichordata into two classes—Enteropneusta and Pterobranchia. But Harmer (1905) added another class Phoronida under Hemichordata. A peculiar pelagic larval form was discovered by Mortensen in 1910. Spengel (1932) named this larva Planctosphaera pelagica.
Phylum Hemichordata Bateson, 1885 or Stomochordata Willey, 1899:
[Gr. Hemi = half, chord = string, having a notochord]; Approx. 85 species.
ADVERTISEMENTS:
Important Features:
1. Hemichordates are bilaterally symmetrical deuterostome coelomates.
2. They are either solitary (enteropneusts) or colonial (pterobranchs).
3. The soft body is divided into 3 regions – Proboscis (Protosome), collar (mesosome) and trunk (metasome) – reflecting internally a tri-coelomate organization.
ADVERTISEMENTS:
4. A preoral buccal diverticulum is considered as the stomochord rather than notochord.
5. The digestive tube is complete, straight or U-shaped.
6. The pharyngeal-slits are present or absent. When present, variable in number (one or more pairs), found in the trunk.
7. A heart and kidney occur in the proboscis (preoral lobe) and are supported by buccal diverticulum (stomochord).
ADVERTISEMENTS:
8. The nervous system is intra-epidermal. The nervous system consists of a dorsal and ventral nerve cord extending to the whole length of the animal. In the collar region, the nerve is dorsal and hollow. This part represents the true chordate feature.
9. Sexes are separate (gonochoristic) but some species like Cephalodiscus nigrescens and C. hodgsoni are hermaphrodite.
10. Gonads are extra-coelomic.
11. Asexual budding also takes place in a few forms (Balanoglossus capensis, Rhabdopleura) and formation of buds occurs in the posterior end of the body.
12. Fertilization is external (enteropneusts) or internal (pterobranchs).
13. Cleavage holoblastic, radial (enteropneusts), but radial and bilateral in pterobranchs.
14. Planktonik, ciliated tornaria larva in some. Tornaria larvae are lecithotrophic (e.g., the larva is non-feeding and completes its developmental stages utilizing its stored reserved food of the egg). Six hypothetical developmental stages are recognised and these are named after renowned zoologists.
1st and 2nd stages are after the names of Muller and Heider. Other stages are Metschnikoff, Krohn, Spengel and Agassiz respectively.
Fossil Record:
Sufficient data have not been collected except Eocephalodiscus which has recovered from the Upper Cretaceous.
ADVERTISEMENTS:
Geographical Distribution:
Hemichordates occur in all seas but may prefer warm and temperate waters. Majority of the species live in the intertidal zone and shallow waters but a few occur at great depths.
Habitat:
Members live in U-shaped burrows in sand, mud or amongst rocks and sea weeds.
The Phylum Hemichordata includes two classes Enteropneusta Gegenbaur, 1870 and Pterobranchia Lankester, 1877. Authors like Marshall and Williams, 1964; Young, 1981; Romer and Parsons, 1986; Barnes, 1987; Ruppert and Barnes, 1994; Anderson, 1988; Pechemik, 2000; and Kardong, 1998, 2000 have mentioned only two classes in their books.
But Hyman (1959) and Barrington (1967, 79) include 3 classes – Enteropneusta, Pterobranchia and Planctosphaeroidea in their classification.
Systematic Resume of the Hemichordata:
Class 1. Enteropneusta [Gr. enteron = intestine ; pneustikos = for breathing ; or Intestine breather] Approx. 70 species.
1. This class includes the acorn worms having vermiform body.
2. They lead a solitary life.
3. There are numerous pharyngeal slits.
4. The intestine is a straight tube and anus is a terminal.
5. The arms are absent.
6. Sexes are separate (gonochoristic). Gonads are numerous and sac-like. Fertilization takes place in sea water.
Occurrence:
They are inhabitants of mucus-lined burrows of intertidal zone of sandy or muddy shores. They are found in the seas of temperate and warm zones. The class includes four families.
Family 1. Protobalanidae (Protoglossidae):
Branchial skeletal pieces free; glomerulus ill-developed; metacoelomic diverticula absent.
Geographical distribution:
British and French coasts.
Examples:
Protoglossus (= Protobalanus, Fig. 2.15A).
Remark:
The family Protoglossidae was created by its discoverers, Caullery and Mesnil (1904), but Burdon-Jones (1956) included the members under the family Harrimaniidae due to lacking of perihaemal diverticula and branchial synapticules.
Family 2. Harrimaniidae:
Branchial synapticula and perihaemal diverticula absent; liver sacs not seen from outside ; simple gonads.
Geographical distribution:
Tropical and cold water of both hemispheres.
Examples:
Harrimania, Saccoglossus (= Dolichoglossus Fig 2.15B).
Saccoglossus (Dolichoglossus) is constructed on the basic enteropneustan plan (Fig. 2.15A) and bears some peculiar individual features. It excavates spiral burrow for living. Saccoglossus pygmaeus is the smallest enteropneust measuring about 2-3 cm. The proboscis is exceptionally elongated and slender in this genus. The posterior fold of the collar hangs like operculum over the anterior end of the trunk.
The glandular collar epidermis exhibits three distinct histological regions. The oesophagus opens to the exterior through four to six pairs of dorsal canals. The dorsal proboscis nerve cord expands into a fan-shaped area in the posterior part of the proboscis.
Family 3. Spengelidae:
Stomochord long and divided into 3 segments; liver sacs absent; pericardial diverticula short; perihaemal diverticula present.
Geographical distribution:
Subtropical, mainly Indo-Pacific and Mediterranean.
Examples:
Spengelia, Schizocardium (Fig. 2.15C), Clandiceps, (Fig. 2.15D) Willeyia.
Family 4. Ptychoderidae:
Perihaemal diverticula and liver sacs present; metacoels segmented; lateral genital wings.
Geographical distribution:
Indo-Pacific, Red Sea, Mozambique to Galapagos Islands, Coast of Japan, subtropical Atlantic.
Examples:
Balanoglossus (Fig. 2.1 5E, Ptychodera (Fig. 2.15F), Glossobalanus.
Ptychodera lava is common on sandy shores of Krusadi Island in the Gulf of Mannar.
Class 2. Pterobranchia [Gr. pteron = fin, branchia = the gills of the fishes or feather gills]. Approx. 15 species.
1. The pterobranchs are colonial, tube dwelling and live in deep sea water.
2. The arms are tentaculated.
3. The body is enclosed in an encasement, secreted by the animals.
4. The alimentary canal is U-shaped and a dorsal anus situated near the mouth.
5. The gill-slits are absent. If present, they are one pair.
6. Sexes are separate or united. Gonads are few in number.
Geographical distribution:
Members were collected in deep water from the seas around Antarctica and off the coast of Japan, but now are collected in shallow waters around Bermuda and in the coasts of Florida.
Habitat:
Normally they are bottom dwellers. They are often found to be attached to other animals such as sponges and bryozoans.
The class includes two families:
Family 1. Cephalodiscidae:
Coenocium present or absent; 4-9 pairs of arms in the lophophore; single pair of gill-slits.
Geographical distribution:
Majority are found in deep waters in Southern hemisphere of which Cephalodiscus indicus has been recorded from Sri Lanka and India, but Atubaria is found in northern waters, specially off the coast of Japan (Sagami Bay).
Examples:
Cephalodiscus (Fig. 2.16A, B) and Atubaria.
Cephalodiscus (Fig. 2.16A & B) live in association and inhabit branched upright tubes secreted by them. The tubes are called coenecium of unknown composition. Foreign particles like sand grains, broken molluscan shells and spicules of sponges adhere to the coenecium. Each tube is occupied by an individual or zooid. All the tubes are fixed permanently on the substratum.
The body is divided into three regions. The proboscis is represented by a flexible disc-like structure which is tilted as the upper lip to conceal the mouth. There are two proboscis pores in Cephalodiscus. The collar bears four to nine pairs of arms. Each arm contains two rows of tentacles which are tipped by glandular knobs.
The trunk is subdivided into two portions; an anterior sacciform part containing the alimentary canal and gonads and a posterior slender part for attachment with the tube. The alimentary canal is U-shaped with the mouth and anus located on the same side of the body. A single pair of gill-slits is present. Tongue-bars and skeletal supports are absent.
The gonads are two in each zooid and are symmetrically placed in the anterodorsal part of the trunk. Asexual reproduction by budding is common. Buds are produced in a localised budding zone near the stalk. Each zooid may possess one to fourteen buds at a time. All the zooids in an aggregation have arisen from a single sexual progenitor by the process of budding.
Atubaria is a solitary pterobranch closely related to Cephalodiscus. This genus, Atubaria, was first described by Sato in the year 1936. The coenecium is absent. The body is about 1-5 mm long. The collar bears four pairs of tentaculated arms.
The distal ends of the second pair of arms are devoid of tentacles and rod-like in appearance. The stalk lacks the adhesive tips and Atubaria clings on the hydroid colonies by its stalk. The pharynx is perforated by a pair of gill-slits.
Family 2. Rhabdopleuridae:
Coenocium formed of ring-like tubes; only one pair of arms; gill-slits absent.
Geographical distribution:
Found in the colder regions of the world, mainly Shetlands (North Atlantic) and South Australia.
Examples:
Rhabdopleura (Fig. 2.17).
Rhabdopleura is a colonial pterobranch (Fig. 2.17A-B).
The zooids secrete small, erect and ring-like tubes of 6-7 mm in height. All the tubes are joined basally by a black stolon. This stolon also connects all the zooids of the colony.
The zooids resemble closely those of Cephalodiscus except that the zooids are very small and measure below one millimeter in length. Only one pair of arms is present in the collar. The gill-slits are totally absent. Single gonad is present and is situated on the right side of the trunk.
Some authors tried to create a third class Planctosphaeroidea for Planctosphaera pelagica, a large-size, transparent spherical larva with extensively branched ciliary bands.
The features resemble with tornaria larva. It seems to be larval form of some unknown abyssal hemichordate and still a debatable issue for the creation of a new class.
Another group, the graptolites (e.g., Dendrograptus) were colonial fossil organisms, known from the Palaeozoic era and classically associated to Cnidaria or Ectoprocta, represent a third extinct class, Graptolithina or Graptolita, placed under Hemichordata, and believed to be related to Pterobranchia. The relationship was disproved by Hyman (1959).
Indian hemichordates:
Collection and Research:
First Punnet (1903) described enteropneust worms from Laccadive and Maldives Islands. Menon (1904) worked on dredged collections off Madras coast. Rao (1955) described some species which are collected from the same area.
Pillay (1950) described the only enteropneust worm, collected from Okha (Gujarat) coast. Dhandapani identified Glandiceps malayanus and Glossobalanus minutus, collected from Andaman and Nicobar Islands and Maldives Islands.
Ptychodera flava occurs in the Krusadai Island, located in the gulf of Manner. The author (Badal Chandra Bharati Goswami) visited Krusadai Island several times with students and has observed the specimen in shallow water of sandy shore. Ptychodera flava lives in U-shaped burrows, lined by a mucous layer.
Rao (1955) recorded Glandiceps bengalensis, Glossobalanus elongates and Ptychodera flava from the Madras (presently Tamil Nadu) coast. Balasubramanyan (1978) described two species of Tornaria from Porto Novo waters in Tamil Nadu.
Saccoglossus has been recorded from Prentice Island, Sundarbans, West Bengal by Singh, B. and A. Choudhury in 1984. It has been collected from mangrove mudflat of Sundarbans, ecosystem.
About 8 species have been recorded from the Madras (at present Tamil Nadu) coast. These are Saccoglossus bournei Menon, 1904; S. madrasensis Rao, 1955; Glandiceps coromandelicus Menon, 1904; G. stiasnyi Rai, 1953; Glossobalanus elongatus spengel; G, minutus and Ptychodera flava Eschscholtz and Glandiceps bengalensis.
