Balanoglossus is a long cylindrical worm like hemichordate animal. It lives in burrows in the sand of intertidal region.

It's body wall is smooth. It is soft, moist and slimy. It is ciliated. This body wall is helpful in locomotion.

The body wall of Balanoglossus will show epidermis, muscles and peritonium.

1. Epidermis : It is made by a single row of cells. These cells are long and columner. They are ciliated. Some cells are gland cells. Some cells are connected with nerve ends and form receptors. They receive sensory stimule.

2. Muscle layer : Below the epidermis muscle layer is present.

i) In the proboscis and collar region circular muscles are on the outerside and longitudinal muscles are towards the innerside.

ii)In the trunk region only longitudinal muscle layer is present.

3. Peritonium : The innermost layer of body wall is called peritonium. It is a thin layer. It is derived from mesoderm.

Functions of Body wall :,

1. It will cover and protect the body organs.

2. It will secrete slime which helps the animal to move in the burrows.

3. It will absorb the external stimule.

4. It will help in locomotion.

Coelom: The coelom of this animal is divided into 5 cavities. It is derived in enterocoelic way.

1. Proboscis coelom : It is a single cavity. It opens out through a pore on the proboscis. It is filled with coelomic fluid. It helps in burrowing.

2. Collar coelom : In this region coelom is represented by two cavities. In this region coelomic cavity is reduced because of the high muscular develop­ment. This coelomic cavities will open into first gill slit.

3. Trunk coelom: In the trunk region two coelomic cavities are present. In these cavities gills, gonads, hepatic caecae etc. are placed.


Skeletal structure of Balanoglossus :

In Balanoglossus different skeletal structures are present. They are

1. Stomochord : It is present in the proboscis. It gives strength to proboscis. It is formed as an extension of Buccal epithelium. This stomochord includes a central cavity. It is considered as a homologous organ with notochord of chor'dales.

2. Branchial skeleton : The gill pouches are supported by chitinous structures. They are primary gill bars and secondary gill bars.

3. Pygochord : On the ventral sideof the gut a longitudinal pygochord is present. Its function is not clearly known.

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