Balanoglossus is a tubicolous or burrowing and exclusively marine animal. It is found in shallow waters between tide marks along the coast of warm and temperate oceans. Balanoglossus is world-wide in distribution.
i. Burrow:
Balanoglossus clavigerus is tubicolous living in U-shaped burrows excavated in the sandy bottom. The burrow has two vertical limbs 50 – 70 cm deep and the two openings 10 – 30 cm apart. The burrows are open at both ends, and spiral coils of faeces like the castings of earthworms may be seen at the posterior opening which is rounded and hidden below the spirally coiled faeces.
The anterior opening of the burrow is funnel-shaped and exposed. In its burrow Balanoglossus lies in a twisted condition but its anterior and posterior extremities are straight. Knight Jones (1952) reported that the animal moves in its burrow with the help of cilia present all over the body.
ii. Protective Device:
The sandy walls of the tube are lined with mucus secreted by the skin mucous glands of the animal. This prevents the collapse of the tube protecting the animal from burial in sand. The animal also secretes a foul-smelling iodoform-like secretion which probably serves as a protective device. In B. biminiensis, 2.6-dibromophenol is secreted which may serve as a disinfectant of the burrow. Iodine is present in the hepatic regions, probably organically bound. One species of Balanoglossus is phosphorescent.
iii. Movements:
Knight Jones (1952) reported that the animal moves in its burrow with the help of cilia present all over the body. Muscular contraction and expansion of the proboscis helps in burrowing. After the tide recedes, the animal protrudes its proboscis out of the burrow to explore the surroundings for food.