Balanoglossus is a marine, soft-bodied, vermiform invertebrate that lives in the coastal areas inside the burrow of sand or mud. It the previously considered a precursor of Chordata and known as primitive Chordata but now it is considered as a separate invertebrates phylum.
Table of Contents
Origin of Name of Balanoglossus:
The name Balanoglossus comes from two Greek words, balanos and glossus, the proboscis of Balanoglossus shows great similarity with the fruit of oak. The Greek word balanos represents the fruit of oak and the word glossus represents tongue, the proboscis looks like a tongue. So the name Balanoglossus is related to the fruit of oak and the shape of the tongue.
The fruit of oak is also known as acorn and for this reason, Balanoglossus is also known as acorn worm, due to the tongue-shaped proboscis Balanoglossus also known as tongue worn.
The collar region and the genital wings of Balanoglossus show great resemblance with ox tongue and for this similarity fishermen generally call Balanoglossus as ox tongue.
Geographical Distribution of Balanoglossus:
Balanoglossus is not confined to any specific geographical area, they are available in almost all areas of the world. But some species are only confined to a certain geographical area, they present in abundance in tropical and subtropical regions.
Habitat of Balanoglossus:
Balanoglossus is marine and they generally live in coastal shallow water in the burrows of sand or mud. But they can also live in deep marine water under the stone or in the seaweeds. Their burrows are U-shaped and the anterior opening of burrows has a funnel-shaped depression and on the posterior end, the opening of burrows remains hidden due to the deposition of fecal matter.
Balanoglossus secret mucous which gives a cementing layer on the interior wall of the burrows. The mucous layers protect the animals against collapsing the sand or mud in the burrows.
The proboscis part is movable in Balanoglossus and helps in making the burrows and it also helps in collecting food present in seawater.
Balanoglossus take food from the seawater in which they live through generations of water current and through the ciliary movement in proboscis region.
Detailed Information on What is Balanoglossus: A Tongue Worm
External Morphology of Balanoglossus:
Balanoglossus have no exoskeleton and their body is very soft, they have a vermiform body shape and the length vary from 10 cm to 60 cm. But some species like Balanoglossus gigas show more than one meter in length, the body of Balanoglossus has three distinct regions, proboscis, collar, and trunk.
Proboscis:
It is the anterior-most conical muscular hollow part of their body. The proboscis has a small coelom that opens outside through proboscis pore on the dorsal side. The narrow part of the proboscis is known as proboscis stalk and there is a U-shaped ciliated region present. This ciliated region is known as the preoral ciliary organ which tests the quality of food before it enters into the mouth cavity.
Collar:
It is a cylindrical short hollow muscular middle part of Balanoglossus. At the base of the collar, proboscis joins with collar through proboscis stalk. The anterior part of the collar has a flap-like structure which is known as collarette. Collarette covers the base of proboscis stalk where the mouth opening presents and inside the collar the mouth opening opens into the buccal cavity. The hollow collar has a coelom that opens outside through two collar pores in the first pair of gill slits.
Trunk:
The trunk is the posterior largest body part of the Balanoglossus which is further divided into three parts, branchiogenital region, hepatic region, and posthepatic region.
Branchiogenital Region:
This portion of the trunk are responsible for respiration and reproduction, it has two lateral flap-like structure known as genital wings. In the branchiogenital region, gill slits are arranged in two rows and the genital wings have gonads. The gill slits are open outside through the gill pores and between the gill slits and gill pore open space is present which are known as gill pouches.
Hepatic Region:
This region has a large number of sacculation which can be seen from the outside and on the dorsal side, it forms hepatic caeca.
Posthepatic Region:
It is the cylindrical narrow posterior most part of the trunk which is also known as the tail and at the terminal end, it has opened for the anus.
Detailed Information on External Morphology of Balanoglossus
Body Wall of Balanoglossus:
In Balanoglossus the body wall is thick and tough due to the presence of a thick muscular layer and the body wall has three distinct layers – epidermis, muscular layer, and peritoneum.
Epidermis:
Epidermal layers are composed of a single layer of columnar secretory cells and below the epidermis nervous tissue present. Below the epidermal cells, the nerve fibers, and nerve cells from the nerve plexus and below the nerve plexus, a basement membrane is present. The basement membrane gives support to the epidermal cells and the nervous tissue, the muscular layer present below the basement membrane is attached to the basement membrane.
Muscular Layer:
The body wall is thick in Balanoglossus due to their thick muscular layer, longitudinal and circular muscle arranged in the body wall. The circular muscle gives control to a different part of the body which requires to be opened or closed.
Peritoneum:
Inside the body wall, the body cavity or coelom is present, the peritoneum act as the lining of the coelom (parietal peritoneum) and it limits the body wall from the coelom.
Functions of Body Wall in Balanoglossus:
- Balanoglossus have no exoskeleton to protect themselves from external mechanical injuries but the tough thick muscular layer give them some extra protection.
- The contraction of longitudinal and circular muscle help them in movement and controlling different body pores opening.
- Body wall epidermal cells secrete mucous which gives a lining to the interior wall of the burrows which prevents the collapse of sand or mud on them.
- The neurosensory cells present on their body wall are able to detect different types of external stimuli and make them aware about their surroundings.
Detailed Information on Body Wall of Balanoglossus
Body Cavity or Coelom in Balanoglossus:
Coelom in Balanoglossus formed from the archenteron of Balanoglossus, so coelom in Balanoglossus is an enterocoelous type. The coelom in Balanoglossus occupies a very small space, the growth of the muscular layer makes the coelom folded and small. The coelom is the proboscis, collar, and trunk region are separated from each other, and a total of five body cavities represent coelom in Balanoglossus. In proboscis, only one small cavity is known as proboscis coelom, in collar two body cavity, and in trunk two body cavity represent coelom.
Proboscis Coelom:
The coelom in proboscis is a very small cavity due to the presence of muscle fibers and connective tissue, proboscis coelom is also known as the protocol. In proboscis coelom central sinus, heart vesicles, and the excretory organ glomerulus are present.
Collar Coelom:
Collar coelom or metacoel is two narrow-body cavities present on the lateral side of the collar. It is the space or cavity between the collar wall and the buccal tube which divides into two by the mid-dorsal and mid-ventral mesentery. Both the collar coelom open outside through the collar canal and collar pore present at first pair of gill sacs.
Trunk Coelom:
Trunk coelom or metacoel is a pair of body cavities present in the trunk region, the trunk coelom presents laterally. The collar-trunk septum separates the trunk coelom from the collar coelom, trunk coelom is the cavity between the trunk wall and the gut. The trunk coelom or cavity is divided into two due to the mid-dorsal and mid-ventral mesentery, in the branchiogenital region, each lateral trunk coelom is further divided into two cavities, the dorsolateral compartment, and ventrolateral compartment.
Coelomic Fluid in Balanoglossus:
All the body cavity in Balanoglossus is not filled with coelomic fluid, only the trunk coelom is filled with coelomic fluid but the collar coelom, proboscis coelom are filled with seawater which gives them turgidity. The body fluids in Balanoglossus have some amoebocyte cells and they perform similar functions as that of white blood corpuscles in us.
Detailed Information on Coelom and Coelomic Fluid in Balanoglossus
Endoskeleton in Balanoglossus:
In Balanoglossus there is no bony or cartilaginous endoskeleton but some structures are stiff and perform the functions of the endoskeleton. So those structures are considered as endoskeletons and we will discuss each of those structures.
- Buccal Diverticulum: It is a hollow stiff tubular structure present in the proboscis coelom which makes some workers confused as notochord. But when it found that this structure are formed from the tissues of buccal cavity then it becomes clear that the so called notochord is actually an extension of buccal cavity. After that it is known as buccal diverticulum instead of notochord or stomochord.
- Proboscis Skeleton: It is a result of thickening of basement membrane in buccal cavity near the buccal diverticulum. Proboscis skeleton is embedded in a fibrous tissue which shows great similarity with the cartilage tissue and it is known as chondroid tissue.
- Branchial Skeleton: In branchiogenital region of trunk two row of gill slits are present and they take part in respiration and generation of water current. Gill slits are supported by M shaped endoskeleton which is formed by joining two U shaped structure.
- Pygochord: In posthepatic region of trunk a cutaneous thickening present which supports the tail region is known as Pygochord.
Detailed Information on Endoskeleton in Balanoglossus
Digestive System of Balanoglossus:
- Digestive system in Balanoglossus is complete, it starts at the mouth opening near the base of proboscis stalk and end at anus at the tip of tail.
- The alimentary canal in Balanoglossus is straight and supported dorsally by mid-dorsal mesentery and in ventral side by mid-ventral mesentery.
- Alimentary canal in Balanoglossus are divided into, mouth, buccal cavity, pharynx, oesophagus, intestine and anus.
- A small circular aperture at the base of proboscis stalk on the ventral side represent mouth opening.
- Mouth is covered by the colarette of collar and the opening of mouth is controlled by circular and radial muscle.
- The mouth open into buccal cavity, the dorsal wall of buccal cavity extend into proboscis which is known as buccal diverticulum.
- Buccal cavity lead into phraynx, the pharynx have two lateral constriction which divides the lumen of pharynx into dorsal respiratory part and ventral digestive parts.
- The dorsal respiratory part of pharynx have two rows of gill slits and in ventral digestion portion different types of digestive glands present.
- Pharynx lead to oesophagus, oesophagus also have partition which starts from the pharynx and in posterior part the lumen of oesophagus become narrow.
- Intestine present in hepatic and posthepatic region of trunk, in hepatic region a large number of sacculation present which is visible from outside and known as hepatic caeca.
- At the end of trunk the tip of tail have circular aperture, anus is controlled by the muscle spincter of circular muscle fibres.
Feeding Mechanism of Balanoglossus:
- Balanoglossus collect food from the sea water, microorganisms, diatoms present in sea water is their food.
- The cilia present in gill slits generate water current which force the sea water to enter into mouth, from mouth sea water pass through buccal cavity, pharynx, gill sac and goes outside through gill pores. The food present in the water current is feed by them and this water current is known as respiratory-cum- food current.
- The proboscis secret mucous which trap the food particles present in sea water and the cilia present on the proboscis give directions to the food particles into the mouth opening.
- The cila present on the proboscis stalk is known as preoral ciliary organ which test the quality of food, if something harmful chemicals present in water the collarete cover the mouth opening and prevent entering the water into mouth.
Detailed Information on Digestive System of Balanoglossus
Respiratory System of Balanoglossus:
In the pharynx the dorsal chamber is responsible for respiration, the pharyngeal wall has two longitudinal rows of gill slits present dorsolaterally. The gill slits open into a sac-like space gill sac and the gill sac opens outside through gill pores.
- On dorso-lateral wall of pharynx have two series of U shaped gill slits arranged along the length of the trunk. Each gill slits supported by M shaped branchial skeleton which keeps the gill slits open, the number of gill slits increase with the age of the animals.
- The blood capillaries present in the gill slits region take part in gaseous exchange, the cilia present on the gill slits generate respiratory-cum-food current and the oxygen for respiration comes from the sea water current.
- The gill slits open into the gill sacs and the gill sacs open outside through gill pores, the water from pharynx flow into gill sac, from gill sacs to outside through gill pores.
Detailed Information on Respiratory System of Balanoglossus
Blood Vascular System of Balanoglossus:
- In Balanoglossus blood vascular system is open type and the coelomic fluid or blood is colourless.
- Respiratory pigment absent in their colourless blood and some white corpuscles present.
- Heart is sac like triangular muscular structure, the ventral muscle of heart vesicles generate pulsation which force the blood to flow through blood vessels.
- Under heart vesicle, central sinus present which form glomerulus in anterior side, in proboscis region four main arteries comes out.
- From glomerulus two efferent glomerular arteries comes out which goes in backwards along the two sides of glomerulus, one mid-dorsal and one mid-dorsal arteries supply blood to the proboscis.
- The two lateral efferent glomerular arteries join to form ventral vessels which run along the ventral side of body and end at the end of the trunk.
- The ventral vessels give branch in its way, in collar region it produces collar vessels, in collar-trunk septum it gives rise ring vessels.
- On dorsal side a vein run from posterior end of the body which collect blood from body wall and other organs and open into venous sinus in collar region. The venous sinus open into central sinus which gives rise to afferent glomerular blood vessels in anterior side in glomerulus glands.
Detailed Information on Blood Vascular System of Balanoglossus
Excretory System of Balanoglossus:
- In Balanoglossus excretion occur through the proboscis gland or glomerulus present in proboscis region.
- The blood from whole body comes in glomerular gland through dorsal vessels and the excretory material present in the blood pass out from the body through the proboscis pore.
Nervous System of Balanoglossus:
- Nervous systems in Balanoglossus is not well developed the nervous tissue present in epidermis, so they have epidermal nervous system.
- Nervous systems have two solid nerve cord, one on the dorsal side, dorsal cord and another one is ventral cord on ventral side.
- The dorsal cord comes outside of epidermis in collar region, the part of dorsal nerve cord in collar region is hollow. The hollow cavity of dorsal cord in collar region is known as neurocoel, this hollow portion of dorsal cord is known as collar cord.
- The dorsal cord present throughout the length of the body and in anterior side it produces a nerve ring in proboscis region.
- The ventral cord do not extend upto proboscis, it gives rise a circumcenteic nerve ring in collar-trunk septum, the circumcenteic nerve ring join with the dorsal cord.
Reproductive System of Balanoglossus:
- Balanoglossus show both asexual and sexual mode of reproduction, but they generally do not multiply through asexual reproduction.
- Balanoglossus show great power of regeneration, one body fragments can give rise to the whole body.
- Male and female are separate and sexual dimorphism is absent in them, only the colour of male and female gonads show different between males and females.
- In branchiogenital region of trunk the genital wings have the gonads in longitudinal rows.
Detailed Information on Balanoglossus: Excretory, Reproductive and Nervous Systems
Affinities of Balanoglossus with Other Groups of Animals:
- Balanoglossus show similarity with Chordata in different aspects, the pharyngeal gill slits in Balanoglossus indicates their relationship with Chordata. But the position and number of gill slits are not exact same as that of Chordata. The notochord like structure present in Balanoglossus is actually buccal diverticulum, the nerve cord in Balanoglossus is solid and not only dorsal.
- Balanoglossus have some similar features as that of Rhynchocephalia, both have vermiform body shape, nerve plexus in body wall, terminal anus Rhynchocephalia do not have dorsal nerve cord.
- Balanoglossus some similar features as that of Phoronida, both have great power of regeneration, show similarity in their larval stage, both have epidermal nervous system.
- Pogonophora also show great similarity with Balanoglossus, both have epidermal nervous system, enterocoelous type of coelom, cardiac sac.
- Annelida and Balanoglossus show some common features, live in burrow, both have preoral proboscis like structure, dorsal heart, resemblance in larval stage.
- Echinodermata and Balanoglossus show great resemblance in both adult stage and larval stage, eye spot, sensory apical plate, enterocoelous type of coelom, both are dueterostome, epidermal nervous system but they also have some important differences.
Detailed Information on
Affinities of Balanoglossus with Chordata
Affinities of Balanoglossus with Rhynchocephalia, Phoronida, and Pogonophora
Systematic Position of Balanoglossus:
- Balanoglossus have a controversial position in animal kingdom due to their close resemblance with different animals groups.
- Based on some features, buccal diverticulum, pharyngeal gill slits they were placed in a subphylum Hemichordata under phylum Chordata.
- But now it becomes clear that the buccal diverticulum is not actual notochord, the nerve cord in Balanoglossus is solid and present on both dorsal and ventral side. The number of pharyngeal gill slits is also many as compared to Chordata.
- So most of the workers refused to place them under Chordata, they prefer to place them in a separate individual invertebrates phylum.
- But due to controversy they yet under Hemichordata, Balanoglossus also show great resemblance with Echinodermata, Pogonophora, Phoronida, Rhynchocephalia so yet the systematic position of Balanoglossus is not clear.
Detailed Information on Systematic Position of Balanoglossus
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