Early developmental stages of Balanoglossus possess a spacious coelom lined with peritoneum. It is enterocoelous in origin. As the adult condition is attained, the coelomic epithelium gives rise to the connective tissue and muscle fibres. Hence the coelomic cavity is obliterated. The coelom as a whole is divided into three parts. These include one cavity in proboscis, the protocoel: two in collar, the mesocoel and two in trunk
In the proboscis, the coelom is a small, unpaired cavity into which several structures like buccal diverticulum, central sinus, heart vesicle, glomerulus etc. project from the base of the proboscis. The proboscis coelom opens out through a dorsal pore at the base of proboscis.
In the collar the coelom consists of two separate sac-like cavities on left and right side due to the presence of dorsal and ventral mesenteries. They communicate with the first pair of gill- sac by a pair of collar pores.
In the trunk the coelom consists of a pair of cavities which are separated by an ncomplete mid-dorsal and complete ventral mesenteries. The trunk coelom is filled with coelomic fluid containing amoeboid corpuscles, the coelomocytes. They originated from coelomic epithelium. According to Spengel (1893), the coelomocytes secrete a membrane around any foreign particle which invade the animal thus behaving like leucocytes.