<P> The gut of the earthworm is a straight tube which extends from the worm's mouth to its anus . It is differentiated into a buccal cavity (generally running through the first one or two segments of the earthworm), pharynx (running generally about four segments in length), esophagus, crop, gizzard (usually) and intestine . </P> <P> Food enters in the mouth . The pharynx acts as a suction pump; its muscular walls draw in food . In the pharynx, the pharyngeal glands secrete mucus . Food moves into the esophagus, where calcium (from the blood and ingested from previous meals) is pumped in to maintain proper blood calcium levels in the blood and food pH . From there the food passes into the crop and gizzard . In the gizzard, strong muscular contractions grind the food with the help of mineral particles ingested along with the food . Once through the gizzard, food continues through the intestine for digestion . The intestine secretes Pepsin to digest proteins, Amylase to digest polysaccharides, Cellulase to digest cellulose, and lipase to digest fats . Earthworms use, in addition to the digestive proteins, a class of surface active compounds called drilodefensins, which help digest plant material . Instead of being coiled like a mammalian intestine, an earthworm's intestine increases surface area to increase nutrient absorption by having many folds running along its length . The intestine has its own pair of muscle layers like the body, but in reverse order--an inner circular layer within an outer longitudinal layer . </P> <P> The earthworm has a dual circulatory system in which both the coelomic fluid and a closed circulatory system carry the food, waste, and respiratory gases . The closed circulatory system has five main blood vessels: the dorsal (top) vessel, which runs above the digestive tract; the ventral (bottom) vessel, which runs below the digestive tract; the subneural vessel, which runs below the ventral nerve cord; and two lateroneural vessels on either side of the nerve cord . The dorsal vessel moves the blood forward, while the other four longitudinal vessels carry the blood rearward . In segments seven through eleven, a pair of aortic arches rings the coelom and acts as hearts, pumping the blood to the ventral vessel that acts as the aorta . The blood consists of ameboid cells and hemoglobin dissolved in the plasma . The second circulatory system derives from the cells of the digestive system that line the coelom . As the digestive cells become full, they release non-living cells of fat into the fluid - filled coelom, where they float freely but can pass through the walls separating each segment, moving food to other parts and assist in wound healing . </P> <P> The excretory system contains a pair of nephridia in every segment, except for the first three and the last ones . The three types of nephridia are: integumentary, septal, and pharyngeal . The integumentary nephridia lie attached to the inner side of the body wall in all segments except the first two . The septal nephridia are attached to both sides of the septa behind the 15th segment . The pharyngeal nephridia are attached to fourth, fifth and sixth segments . The waste in the coelom fluid from a forward segment is drawn in by the beating of cilia of the nephrostome . From there it is carried through the septum (wall) via a tube which forms a series of loops entwined by blood capillaries that also transfer waste into the tubule of the nephrostome . The excretory wastes are then finally discharged through a pore on the worm's side . </P>

What are the hearts of a worm called