The excretory system of Fasciola hepatica is concerned with excretion as well as osmoregulation. It consists of a large number of flame cells or flame bulbs or protonephridia connected with a system of excretory ducts.
The flame cells, supposed to be modified mesenchymal cells, are numerous, irregular in shape bulb-like bodies found distributed in the mesenchyma throughout the body of Fasciola. The distribution pattern of flame cells follows a specific pattern referred to as ‘the flame cell pattern’ (Faust, 1919).
The flame cells are characteristic, each has a thin elastic wall with pseudopodia-like processes, a nucleus and an intracellular cavity having many long cilia arising from basal granules. In living condition, the cilia vibrate like a flickering flame, hence, the name flame cell.
There is an excretory pore at the posterior end from which arises a longitudinal excretory canal, from this arise four main branches, two dorsal and two ventral, which subdivide into numerous small capillaries which anastomose; the capillaries are continued into the intracellular cavity of flame cells. The longitudinal excretory canal is non-ciliated but the capillaries are lined with cilia.
Process of Excretion
The excretory wastes, generally fatty acids and ammonia, are diffused from surrounding mesenchyma into the flame cells and finally collected into their intracellular cavities.
The vibrating movement of the cilia causes the flow of wastes from the intracellular cavities of flame cells into the excretory ducts and then into the main excretory canal and finally to the exterior through excretory pore by hydrostatic pressure.
Such an excretory system of flame cells and canals or ducts of various orders with no internal opening and leading to an excretory pore which opens to the exterior is spoken of as a protonephridial system which is excretory but its main function is to regulate the amount of fluid in the animal’s body.