Habitat[ edit ] Balanoglossus is a tuberculos[ check spelling ] or burrowing and exclusively marine animal. It is found in shallow waters between tide marks along the coast of warm and temperate oceans. Balanoglossus clavigerus is a U-shaped excavated in a sandy bottom Affinities with Nemertinea[ edit ] 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.
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Affinities and Systematic Position of Balanoglossus 1. Delle Chiaje first recorded the animal from the Naples region in and gave the name Balanoglossus clavigerus. Hyman stated that the balano part does not derive from the Greek balanos, meaning an acorn, in reference to the shape of the proboscis, but from the barnacle genus Balanus, to which Delle Chiaje found some resemblance in his specimens.
Delle Chiaje also reported that the animal was called ox- tongue by the local fishermen. Geographical Distribution of Balanoglossus : Balanoglossus is a marine animal and inhabits all the seas of the world. About 20 species of Balanoglossus have been recorded in the tropical and sub-tropical seas. Indies ; B. Habit and Habitat of Balanoglossus : Balanoglossus is a burrowing animal.
It is mostly an inhabitant of intertidal zone, although a few descend to about m of the sea. Balanoglossus excavates its own burrow in sand or sandy mud.
The burrow of Balanoglossus clavigerus is U-shaped with two openings 10 to 20 cm apart Fig. The anterior end of the burrow is funnel-like and the posterior end is round. The anterior vertical part of the burrow may be branched. When the tide starts ebbing, the proboscis protrudes out of the burrow to explore the surface.
A constant secretion of mucus over the body causes adhering of sand-grains over the body in the form of a cast. Balanoglossus emits a peculiar odour resembling that of iodoform. This is regarded as a protective device. The proboscis becomes elongated by the contraction of circular muscles and the shortening of the proboscis is caused by the contraction of its longitudinal muscles. The two sets of antagonistic muscles are so co-ordinated that when one set contracts the other set relaxes, and vice versa.
Balanoglossus is less sensitive in response to external stimuli. It takes sand-containing diatoms, protozoans and many other micro-organisms. The larval form Tornaria larva of Balanoglossus is a free- swimming planktonic form which exploits the natural resources of the sea.
The length of the body usually ranges from cm. But Balanoglossus aurantiacus of the south eastern coast of the United States attains about 1 meter in length. The largest acorn worm Balanoglossus gigas reaches is more than 2. It can construct a burrow up to a depth of 75 cm or more. The body is divisible into three major parts—the proboscis or protosome, collar or mesosome and trunk or metasome Fig.
The trunk is further subdivided into an anterior branchiogenital region, a median hepatic and a posterior caudal or post-hepatic region. The proboscis narrows posteriorly to a proboscis stalk. It is hollow and communicates with the exterior by a single proboscis pore situated on the left side of the proboscis.
The proboscis is anterior to the collar. The collar cavity is paired and opens to the exterior by two apertures. The collar houses the mouth on the ventral side Fig. Trunk: The trunk is rather long. Double rows of gill-slits or pharyngeal slits are present on the dorsal surface of this region. The gill-slits increase in number as the animal grows older. The caudal region is more or less uniform in diameter and is marked externally by annulations.
The trunk bears a mid-ventral and a mid-dorsal ridge which contain the longitudinal blood vessels and the longitudinal nerves. The hepatic region tapers posteriorly and terminates into the anus. Body Wall of Balanoglossus : The body wall is composed of a ciliated epidermis.
The ciliated cells are mostly tall and slender. Gland cells are also present in the epidermis. There are three types of gland cells: a Reticulate gland cells, where the cytoplasm contains mesh-like reticulum; b Mulberry gland cells, having coarse granules and c Coblet cells, which are filled up with fine granules.
The cytoplasm is homogeneous Fig. The cell body is divided into two parts— a distal flask-shaped expanded portion and a proximal slender stalk. The stalk extends up to the base of the epidermis. A very thin connective tissue layer is present between the nervous layer and the epidermis. The nervous layer is separated internally by a well-developed basement membrane. Beneath the basement membrane lies the muscular sheath.
In the trunk region only longitudinal muscle fibres are recognised. The muscle fibres are of smooth variety. The inner side of the body wall is lined with parietal coelomic epithelium. The thick- walled anterodorsal hollow prolongation of the buccal cavity buccal diverticulum see Fig. It contains a cavity which opens into the pharynx.
So it is called stomochord instead of notochord. It extends into the proboscis cavity and is covered over by a sheath of basement membrane. It is composed of tall vacuolated endodermal cells. But topographically as well as developmentally this structure is quite unlike the true notochord of other chordates.
The buccal diverticulum, according to most of the modern workers, is a preoral extension of the buccal cavity. Current studies using gene expression similarly fail to find homology between stomochord and notochord.
Proboscis Skeleton: The basal part of the proboscis stalk and the roof of the buccal cavity are supported by a skeletal structure, called proboscis or nuchal skeleton. The median plate is situated on the proboscis stalk and it occupies a median position between the buccal diverticulum and the buccal epithelium. The median plate bears a mid-ventral keel. The horns diverge posteriorly to support the roof of the buccal cavity. One prong of the skeleton is present in the septum and the other lies in the tongue Fig.
The skeletal pieces of the adjacent septa are connected by transverse rods, called synapticulae. Pygochord: In the posterior part of the intestine of some Ptychoderidae e. It may be solid or hollow, and may contain isolated cavities. The function of this structure is still unknown. Locomotion in Balanoglossus: Balanoglossus is a sluggish animal and spends most of its time in burrows. The cilia present on the epidermal cells of the body help in locomotion.
The animal moves by alternate lengthening and shortening of the body produced by the action of muscle fibres. The relative position of the buccal diverticulum and the proboscis skeleton make the proboscis and collar stiff. The stiffness of the proboscis and collar helps in burrowing through the sand. Balanoglossus utilizes the cilia for normal movement and the muscles are employed only during emergency escape. Coelom in Balanoglossus: Balanoglossus possesses a spacious coelom.
The original cavity of the coelom is greatly reduced by the presence of fibrous network and muscle fibres. In some regions the coelomic epithelium is found to be lacking due to its transformation into other tissues. Coelomic sacs are present in the body of Balanoglossus. The proboscis coelom protocoel is an unpaired cavity. The buccal diverticulum, central sinus, heart vesicle and glomerulus constitute the proboscis complex.
The proboscis coelom opens to the exterior through a proboscis pore by way of tubular proboscis canal. All species under this genus have one proboscis pore and canal, but Balanoglossus australiensis has two such canals and pores. Such occurrence is to be treated as an individual variation. The collar coelom Mesocoel is shifted as a pair of sacs between the buccal tube and the collar wall.
Absence of septum between the proboscis and collar is an important feature in the anatomy of Balanoglossus. The dorsal and ventral mesenteries separating the collar coeloms are mostly incomplete. The collar coeloms communicate to the exterior by the collar tubes and collar pores. Like that of collar coeloms, the trunk coelom Metacoel is divided into two lateral halves by the presence of dorsal and ventral mesenteries.
The collar coelom and the trunk coelom are separated by collar- trunk septum. Each trunk coelom projects into the collar in the form of two evaginations, one is digit form and called perihaemal space and the other is peribuccal space. The peribuccal space is less developed in this genus. The evaginations push the collar-trunk septum into the collar and thus make the collar coelom complicated. As the proboscis coelom and the trunk coelom have external communications, the coelomic fluid contains largely sea water.
But the coelomic fluid present in the trunk coelom has amoeboid coelomocytes and the fluid coagulates in histological fixatives. Digestive and Respiratory Systems of Balanoglossus : The alimentary canal is a straight tube running between the mouth and anus. It is peculiar because musculature is absent. The mouth is a large opening situated on the ventral side of the collar. Formerly, the mouth was believed to remain permanently open.
Quick Notes on Balanoglossus