Balanoglossus is a marine animal found almost all over the world. It is burrowing animal found living at the bottom of shallow waters although some species are also found at the depth of 2,500 fathoms.
Balanoglossus is worm-like animal with soft, elongated cylindrical body. They range in length from 3 cm to as much as 2.5 m (B. gigas). They are bilaterally symmetrical animals.
The body surface is uniformly ciliated and is uniformly covered with mucus. The body is divided into three parts: (1) proboscis (protosome, (2) collar (mesosome), and (3) trunk (metasome).
Division of body
Externally the body is divisible into three regions : The proboscis or protosome, collar or mesosome and trunk or metasome.
It is the anterior-most part of the body and is conical in form, tapering anteriorly. Posteriorly, it continues into a narrowed proboscis stalk which is mostly concealed under the collar and is continuous with the inner surface of the dorsal wall of collar. Below the stalk base the proboscis bears a “U” shaped cilialed depression called the preoral ciliary organ which is a chaemoreceptor. Proboscis encloses protocoe! or coelom of proboscis. The proboscis coelom opens out through the proboscis pore situated mid-dorsally near its base.
It is the middle, short and thick belt-like pan of body, lying behind the proboscis. Its surface is often marked with circular grooves or elevations. The anterior funnel-like part of collar that encircling the proboscis stalk is called collarette. Vcntrally, below the proboscis stalk, the collarette encloses a wide aperture, the mouth. The collar is well demarcated from the trunk behind by a circular constriction. The collar has thick musculature and encloses the collar coelom. Sometimes the collar coelom is divided into left and right parts by drosal and ventral mesenteries. The collar coelom opens by a pair of collar pores into the first pair of branchial sacs.
It is the posterior and largest part of the body. It is somewhat flattened and usually shows superficial annulations. It is also marked by mid-dorsal and mid-ventral longitudinal ridges. It is differentiated into three regions: the anterior branchio-genital region, middle hepatic region and posterior abdominal region The branchio-genital region is distinguished by the presence of a longitudinal row of gill- pores on either side of mid-dorsal ridge. Each row of gill- pores is mounted on a prominant ridge-like elevation. The sides of branchio-genital region are thin and leaf-like and are referred to as genital wings, containing the gonads. The gonads open out through gonopores which are microscopic apertures. The genital wings are usually curved and folded as the dorsal side coming close together in the median line thereby concealing the gill-pores.
The hepatic region is marked by numerous small, paired, transverse folds, the hepatic caeca, on the dorsal side. It is dark brownish or greenish in colour. The post-hepatic region or abdominal region gradually tapers behind and bears a terminal anus.