HEMICHORDATA-PTEROBRANCHIA

Solitary or colonial,sessile and tubicolous animals living inside secreated chitinous tubes
Body short,compact,with stalk for attachement..
Proboscis shield-like.
Collar bearing ciliated arms.
Alimentary canal u-shaped
Anus dorsal lying near mouth
Ciliary feeding..
Gill-slits one pair or absent.
Sexes separate or united.
Gonads 1 or 1.
Asexual reproduction by budding in some.

HEMICHORDATA-CLASSIFICATION

Hemichordata includes about 80 species include Enteropneusta and Pterobranchia . 

Enteropneusta

Solitary ,free-swimming or burrowing animals commonly called the "acorn" or "tongug worms".

Body elongated,vermiform.

Proboscis cylindrical and tapering.

Collar without ciliated arms(lophophore).

Alimentary canal straight.

Mouth and anus at opposite ends.

Filter feeding.

Several pairs of u-shaped gill-slita.

Sexes separate.

Gonads numerous,sac-like.

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HEMICHORDATA-CHARACTERS

It include marine,solitary or colonial and mostly tubicolous.
Body is soft, fragile,vermiform,unsegmented,bilaterally symmetrical and triploblastic.
Body is divided into three regions proboscis,collar,and trunk.
Body wall of a single-layered epidermis with mucuous glands.
Coelom is enterocoelous, usually divided into protocoel,mesocoel and metacoel.
Digestive tube complete,straight or u-shaped.
Ciliary filter feeders.
Circulatory systems simple and open.
Excretion by a single proboscis gland.
Nervous system primitive.Dorsal collar nerve cord hollow.
Reproduction mainly sexual.
Fertilization external in sea water.Leads to tornaria larva.

Balanoglossus-AFFINITES WITH CHORDATA

In 1885 WILLIAM BATESON proposed closer affinites with chordata and non-chordata.The resemblances  shows on the presence of a notochord,nerve chord, and pharyngeal gill-slits(pharyngotremy).In cephalochordata and urochordatd the structure and function of pharynx and branchial apparatus are similar.In Branchiostoma the origin of coelom is enterocoelic.soHemichordata had been considered as subphylum of the chordata.

Balanoglossus-AFFINITES &SYSTEMATIC POSITION OF BALANOGLOSSUS

The group ENTEROPNEUSTA, to which BALANOGLOSSUS belongs,was established by GEGENBAUR in 1870.BATESON(1885) proposed the name Hemichordata in place of enteropneusta. Due to their peculiar anatomical organisation and embryology, the hemichordata (balanoglossus) have been closer to the chordata and non-chotdata.

Balanoglossus-METAMORPHOSIS

The larva swims freely,leads a plantonic life feeding on minute organisms, and metamorphoses into an adult worm. During metamorphosis, the size is reduced probably due to loss of water. Transparency, ciliary bands, sensory cilia and eye spots are lost. Body becomes differentiated into roboscis,collar and trunk by the aearance of two constrictions, and the trunk region is elongetedd. The hydroore ersists as proboscis pore, and the buccal diverticulum and gill-slits appear as outgrowths of the alimentary canal. The animal now sinks to the bottom to lead a benthonic life as an adult.

Balanoglossus-TORNARIA LARVA

Tornaria larva was first described by J.MULLER in 1850 the Balanoglossus clavigerus.size 3 mm in size.It is glossy in appearance with an oval body having ventral mouth near the equatorial plane of the body, a posterior terminal anus and gut differentiated into an oesophagus, stomach and intestine. The cilia form two bands on the body surface. The anterior ciliaryn band or cirumoral band takes up a winding course over the preoral surface and forms a postoral loop;its cilia are short and serve to collect food. The posterior ciliated band or telotroch occurs as a ring in front of the anus;its cilia are long and serve as locomotor organs. At the anterior end is an apical plate of thickened epidermal cells,which bears a pair of eye spots or ocelli and a tuft of sensory cilia called apical tuft or ciliary organ. The protocoel(proboscis coelom) in the form of a thin-walled sac,is present and opens to the exterior through a hydropore(proboscis proe).To the right of the hydropore lies a pulsating heart vesicle.The collar and trunk coeloms appear in the older larva.

Balanoglossus-LARVAL DEVELOPMENT

During the formation of the rotocoel, the inner end of the early gut moves towards the ventral surface and opens to outside through a mouth.The gut is now regionated into the oesophagus, stomach and intestine.The intestine opens to outside through an anus, formed at the side of the closed blastopore.The embryo become unfifomly ciliated and escape from the egg membrane to lead a free swimming larval life. It is called tornaria.

Balanoglossus-prelarval development

The prelarval development is similar to that of Branchiostoma. The zygote, undergoes cleavage which is holoblastic, almost equal and of the radial type. It result into a sphere of blastomeres, the morula. The morula undergoes into blastomeres formation and form of a single-layered,hollow and spherical embryo,the blastula or coeloblastula. Its cavity called blastocoel. Blastula result in about 6-15 hours after fertilization.Within 12 to 24 hours,an invagination starts in the blastula which deepens to form to form the archenteron that opens to outside through a blastopore.Soon the blastopore closes and the embryo,now called gastrula.The tip of archenteron is pinched off as a coelomic vesicle called the protocoel.Thus origin of coelom isenterocoelic.The remaning portion makes gut.The protocoel becomes trigular in shape.The hydropore, towards the dorsal side of the embryo.The protocoel and hydropore are the proboscis coelom and proboscis pore.The collar and trunk coelom arise as solid invagination of the hindgut,independent of the formation of protocoel.

Balanoglossus-Development

Development is indirect with a pelagic larval stage, called tornaria.
Fertilization-During breeding season(May to june), mature sperms and ova escape into the surrounding sea water where fertilization takes place. Thus, fertilization is external.

Balanoglossus-sexual reproduction

Sexes are separate. Males and females cannot be identified externally except for the differences in colour of the ripe gonads in the living specimens.Testes of male and ovaries of female are similar. They are sac-like bodies occuring in saveral longitudinal rows in the genital wings, on either side of the alimentary canal.Each gonads has a narrow neck or ductule that opens out through a gonopore. All gonopore are situated outer to the gill pores. Ova are small with poor yolk content.

Balanoglossus-Regeneration

Balanoglossus shows great power of regeneration. Proboscis, collar and isolated pieces from rrunk can regenerate the lost part of the body completely.

Balanoglossus-Reproductive system

Asexual reroduction-Asexual reproduction is rare in enteropneusts. Gilchrist(1923) has described it in one species, Balanoglossus capensis, which lacks heatic caeca. During summer the young worms (juveniles) cut off small iecies from tail end, each regenerating into a comlete sexual adult in winter.

Balanoglossus-Veins

Blood body wall,gut wall and branchial apparatus (efferent branchial vessels)s collected by a single median dorsal vessel which runs through the dorsal mesentery, from posterior end upto the collar. It has muscular and contractile walls and the blood flows in it forwards. At the anterior end of collar, the dorsal vessel dilates a little to form a venous sinus. The latter receives a lateral proboscis vein from each side of proboscis and then opens into the central sinus.

Balanoglossus-Arteries

Blood from the glomerulus is carried away by four arteries.Two arteries, a mid-dorsal proboscis artery and a mid-ventral proboscis artery, supply the proboscis. Two, called efferent glomerular arteries, run backward along the two sides of buccal diverticulum encircle the buccal tube as peribuccal vessels and unite in a single longitudinal ventral vessel that runs up to the posterior end of the body through the ventral mesentery..The ventral vessel gives out a ventral collar-vessel to the collar, a ring vessel to the collar-trunk septum and an afferent branchial artery to each gill septum in which it bifurcates to supply two adjacent tongue bars. All these branches break up into a system of sinuses in their respective structures.The ventral aessel has muscular contractile walls and the blood in it flows backwards.           

Balanoglossus-Central sinus and heart vesicle

The central sinus is a small elongated non-contractile sinus situated in the proboscis just above the buccal diverticulum.It is a closed triangular cardiac sac or heart vesicle. Its ventral wall is muscular and contracts rythmically thereby producing pulsation in the central sinus that help in the circulation of blood.The central sinus receives blood from collecting vessels that opens into its posterior end. It pumps its blood into several afferent vessels which form a plexus in the proboscis gland or glomerulus, lying in front of it. In glomerulus the blood gets rid of its excretory wastes.

Balanoglossus-BLOOD VASCULAR SYSTEM

The blood vascular system of Balanoglossus is of the open or lacunar type.It consists of:a colourless blood,a central sinus and a heart vesicle ,distributing vessles or arteries and sinuses,and collecting vessels or veins.
BLOOD-The blood is a colourless fluid, contaning few white corpuscles which are possibly detached endothelial cells. A respiratory pigment is probably absent. The functioning of the circulatory system is not properly understood.

Balanoglossus-Sense organs

Sense organ of Balanoglossus are simple and comprise:Neuro-sensory cells in the epidermis of proboscis and anterior part of collar, sensitive to touch and light.Preoral ciliary orgsn situated ventrally at the base of proboscis, which is a chaemoreceptor.

Balanoglossus-Nervous system

The nervous system is of primitive type resembling that of coelenterates and echinoderms.Nervous layer is thickened along definite strands to form synapses with the processes of nerve cells.Nervous layer is thickened along definite strands to form two main nerve cords, one mid-dorsal and other mid-ventral,which run along the entire length of the trunk. Ventral cord extends upto collar-trunk septum where it is connected with the dorsal cord by a circular strands, called circumenteric nerve ring. Dorsal cord extends anteriorly upto the base of proboscis whereit is connected with another circular strands called anterior nerve ring.Dorsal cord leaves the epidermis and traverses the collar coelom as collar cord.The collar cord contains a cavity called neurucoel.

Balanoglossus-Excretory system

The excretory organ in glomerulus or proboscis gland lying in front of the central sinus and projecting into the proboscis coelom. It is made up of several blind tubular projections formed,by the peritoneum covering the buccal diverticulum,central sinus and heart vesicle. The tubular projection contain blood confluent with that of central sinus. The excretory peritoneal cells of glomerulus contain yellow or brown granules, probably of excretory substances. From glomerulus the excretory substances pass on into proboscis coelom and finally to the exterior through the proboscis pore.

Balanoglossus-Mechanism of respiration

The lateral cilia lining the gill-slits set up a food-cum-respiratory current of water. It enters the pharynx through mouth, then passes through gill-slits into the branchial sacs and finally leaves through the gill pores. The tongue bars are richly vascular and participate in gaseous exchange. The blood of their capillary networks takes up the oxygen dissolved in water and returns carbon dioxide to it.

Balanoglossus-Branchial sacs.

Gill-slits do not open directly to outside.Each gill-slits opens into a gill pouch called branchial sac, which lies between the bodywall and the pharynx.Each branchial sac in turn opens to the exterior by a small independent gill pore.In B.misakiensis the first four pouches become united .The collar coelom also communicates with the common branchial sac of its side through a collar canal.The gill -pores are visible externally in two longitudinal rows one on each side of the mid-dorsal ridge in the branchiogenital region of the trunk.

Balanoglossus-Respiratory System

The respiratory apparatus of Balanoglossus comprises:
The branchial sacs that open out through gill-pores.
The branchial portion of pharynx bearing gill-slits.
Branchial pharynx.Two lateral longitudinal parabranchial ridges divide the pharyngeal cavity into a ventral digestive portion and a dorsal respiratory or branchial portion.Dorso-laterally,on each side,the branchialportion is perforated by a longitudinal series of numerous U-shaped opening, the gill-slits.Their number varies and increases as the animal growa older.To start development a gill-slit is a broad oval slit. Later, a hollow projection of dorsal pharyngeal wall, called tongue bar,grows into the making it U-shaped.A tongue bar is connected with adjacent gill septa by short transverse or horizontal connections, the synapticula. The development and arragement of gill-slits is identical with that found in Branchiostoma.The gill-slits are richly lined by cilia, called lateral cilia.

Balanoglossus-Food,feeding and digestion

Balanoglossus is a "ciliary feeder".Its food comprises microscopic organism and organic particles present in water and the bottam sand in which it makes its burrows.The lateral cilia lining the gill-slits set up a current of water which enters through the mouth,takes its course through the boccal cavity,pharynx,gill-slits and branchial sacs, and leaves through the gill pores. This is the respiratory-cum-food current.The u-shaped pre-oral ciliary organ, at the base of proboscis staiks,tests the quality of food and water entering the mouth.In pharynx, the food moves through the ventral digestive portion.The exact process of digestion in Balanoglossus is not know.Undigested substances, along with sand and slits, pass out through the anus as"castings".

Balanoglossus-Anus

Posterioly,the intestine opens to the exterior by a terminal circular aperture,the anus at the tip of the trunk.It is often surrounded by a sphincter muscle.

Balanoglossus-Intestine

It occupies the hepatic and post-hepatic region of trunk.The hepatic region of the intestine is highly vascular.Its epithelial cells are dark green or dark wall forms numerous sacculation called hepatic caeca.The intestine wall lies in close contact with the body wall,so that the intestine sacculation correspond with those of the body wall.The post-hepatic region of intestine is connected with the ventral body wall by the pygochord described earlier. It is a simple and straight tube bearing a dorso-lateral grooves lined by tall epitheliar cells with long cilia.

Balanoglossus-Oesophagus

Behind the last pair of gill-slits the pharynx continues into the short oesophagus.The dorsal and ventral division of pharynx continue for some distance into oesophagus.IN THIS REGION ,THE dorsal part is calles postbranchial canal which posses thick,folded and glandular epithelium.The posterior part of oesophagus reduces in diameter and has deeply furrowed epithelium.

Balanoglossus-PHARYNX

It lies in the branchial region of the trunk.Externally its wall bears a longitudinal constriction along each lateral side. These lateral constrriction project into its lumen as ridges,called parabranchial ridges,consisting of tall columnar cells.These ridges incompletely divide the pharynx into a dorsal respiratory or branchial portion and a ventral digestive portion.The dorsal branchial portion is perforated dorso-laterally by two rows of U-shaped gill-slits ,and is concerned with respiration.The ventral digestive portion,lined with ciliated epithelium with gland cells,helps in food concentration.