ULTRASTRUCTURE OF THE GASTRODERMIS AND POSTERIOR GUT POCKETS OF CONCINNOCOTYLA-AUSTRALENSIS (PLATYHELMINTHES, MONOGENEA, POLYSTOMATIDAE) AND COMPARISON WITH ANOTHER POLYSTOMATID, NEOPOLYSTOMA-SPRATTI

While the majority of polyopisthocotylean monogeneans feed on blood, s ome polystomatids infecting chelonians do not. This study examined the gastrodermis of two polystomatids, one - Neopolystoma spratti Picheli n, 1995 - from the conjunctival sac of a chelonian and the other - Con cinnocotyla australensis (Reichenbach-Klinke, 1966) - from the oral ca vity, gill arches and primary gill lamellae of a fish, and also found no evidence of blood feeding. However, the gastrodermal architecture i n both species basically resembles that found in blood feeding polyopi sthocotyleans, with alternation of lamellated digestive cells and an i ntervening syncytium. In C. australensis, oesophageal secretion appear ed to be responsible for initial extracellular digestion and this was followed by pinocytotic uptake of partly degraded material in pits bet ween the numerous apical lamellae of digestive cells. Posterior dorsol ateral gut pockets unique to C. australensis were shown to be blind sa cs separated from the external environment by a narrow cytoplasmic bri dge, composed of a thin layer of tegument apposed to a thin layer of p ocket syncytial epithelium. The pockets are lined with greatly folded syncytium and set in a ''capsule'' of tissue in which numerous protone phridial flame cells are embedded. It is suggested that the pockets ha ve an osmoregulatory function related to the particular environment an d evolutionary history of the host, the primitive lung fish (Dipnoi) N eoceratodus forsteri (Krefft, 1870).