Write general characters of Balanoglossus.

Hint: Balanoglossus, commonly called acorn worm is an ocean-dwelling Hemichordate. It is an exclusively marine organism found in shallow waters in coastal regions of warm oceans. Their body is divided into proboscis, collar, and trunk. It is a burrowing animal. They are basically ciliary feeders. Development can be direct or indirect.

Complete answer:
- Balanoglossus has a worm-like, brittle and soft body.
- Their body cavity is enterocoelous.

- They have a complete alimentary canal present in the digestive system.
- They respire by gill or body surface. Their respiratory pigment is vanadium in their blood.
- They have open circulation with colorless blood and amoeboid corpuscles.
- They possess true notochord. They do not have post- anal tail.
- Excretion is done by a single glomerulus commonly called the proboscis gland.

Additional information:
- Central nervous system is just like non chordates. The brain is present in the form of a nerve ring. Primitive nervous consists of the subepidermal nerve plexus.
- Cleavage is holoblastic in balanoglossus.
- Its food comprises minute organic substances, microscopic organisms, mud, and sand.
- It uses its muscular proboscis and collar to burrow sand and mud.
- In balanoglossus development can be both direct as well as indirect.
- If development is indirect, the larva is called Tornaria.
- Hemichordata is divided into two classes- Enteropneusta and Pterobranchia. Balanoglossus comes under Enteropneusta.
- They lack two major characteristics of chordates- notochord and a post- anal tail.
- in hemichordates, a notochord like structure is found known as stomochord.it is a hollow outgrowth arising from a buccal cavity called a buccal diverticulum in the proboscis.

Note: Modern taxonomists do not include hemichordates in the chordate phylum. Hyman kept in separate phylum hemichordata in invertebrates. Hemichordata is the connecting link between non-chordates and chordates.