Not only is the coelom thus subdivided, but the enteron (gut, alimentary canal, digestive tube) itself shows indications of three main subsections in continuity with one another: - (I) proboscis-gut (Eicheldarm, stomochord, vide infra); (2) collar-gut (buccal cavity, throat); (3) truncal gut extending from the collar to the vent.
In some species (Spengelidae) there is a long capillary vermiform extension of the stomochord in front.
The nuchal skeleton is a non-cellular laminated thickening of basement-membrane underlying that portion of the stomochord which lies between the above-mentioned pouches and the orifice into the throat.
It seems likely that the coelomic pore-canals were originally excretory organs, but in the existing Enteropneusta the pore-canals (especially the collar canals) have, as we have seen, acquired new functions or become vestigial, and the function of excretion is now mainly accomplished by a structure peculiar to the Enteropneusta called the glomerulus, a vascular complex placed on either side of the anterior portion of the stomochord, projecting into the proboscis-coelom.
The blood, which is a non-corpuscular fluid, is propelled forwards by the contractile dorsal vessel and collected into the central bloodsinus; this lies over the stomochord, and is surrounded on three sides by a closed vesicle, with contractile walls, called the pericardium (Herzblase).