The coelom is enterocoelous having been formed as outgrowths of the enteron. Corresponding with the three body regions the coelom is divided into three portions which are completely separated from each other by septa. The coelom is lined with coelomic epithelium or peritoneum.
But enteropneusts are peculiar in that their coelomic epithelium has connective tissue and muscle fibres which fill much of the original coelomic cavities, and a distinct peritoneal lining has disappeared, moreover the coelomic musculature largely replaces the body wall muscles.
The three parts of the coelom are an unpaired proboscis coelom, a pair of collar coeloms, and a pair of trunk coeloms:
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1. Proboscis Coelom:
The proboscis coelom or protocoel is a single space in the proboscis which is largely occupied by muscles and connective tissue and a few structures like buccal diverticulum, glomerulus and central sinus or heart. Dorsally, towards the posterior side, the proboscis coelom is divided by a dorsal mesentery into right and left ventro-lateral compartments which extend into the proboscis stalk; the left compartment is larger than the right and communicates with the exterior through the proboscis pore situated mid-dorsally at the base of the posterior stalk. Ventrally the proboscis coelom is divided by a ventral mesentery into right and left ventro-lateral compartments which are continuous behind the mesentery.
2. Collar Coelom:
The collar coelom or mesocoel has two cavities lying side by side in the collar, one on each side between the collar wall and buccal cavity. The two cavities are partitioned by incomplete mid-dorsal and mid-ventral mesenteries.
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The collar coelom does not communicate with the proboscis coelom, but posteriorly, its each cavity opens into the first gill sac of its side by a canal called collar canal. Each collar coelom opens to the exterior by a collar pore. The collar coelom is greatly obliterated by the collar musculature and connective tissue.
3. Trunk Coelom:
The trunk coelom or metacoel has two closed cavities lying between the body wall and alimentary canal. The two cavities are separated by an incomplete dorsal and a complete ventral mesentery. In the branchio-genital region each cavity is further divided by a lateral septum into a dorso-lateral and ventro-lateral compartment. The trunk coelom is separated from the collar coelom by a collar-trunk septum. The trunk coelom is obliterated by the trunk musculature.
Coelomic Fluid:
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The proboscis and collar coeloms communicate with the exterior and get filled with sea water through their pores, which keeps them turgid. The trunk coelom is filled with a watery coelomic fluid having amoeboid coelomocytes. The coelomocytes originate from the coelomic epithelium. Each coelomocyte possesses a single large vacuole. According to Spengel, they behave like leucocytes by secreting a membrane around any foreign body that may invade the animal.