SWALLOWING OF SEA-WATER AND ITS ROLE IN EGESTION IN THE MONOGENEAN ENTOBDELLA-SOLEAE, A SKIN PARASITE OF THE COMMON SOLE (SOLEA-SOLEA), WITH OBSERVATIONS ON OTHER MONOGENEANS AND ON A FRESH-WATER TEMNOCEPHALAN

Swallowing sea water plays an important part in the process of egestio n in several, epidermis-feeding, monogenean parasites from the skin an d gills of fishes. In Entobdella soleae, from the skin of the common s ole, Solea solea, the retracted pharynx acts as a peristaltic pump, fo rcing sea water into the branched and blind-ending intestine, inflatin g the gut diverticula and providing a suspension medium for debris fro m the digestive process. This secondary role for the pharynx has not p reviously been recorded. Inflation of the gut extends the muscles lyin g beneath the gastrodermis, and possibly neighbouring parenchymal musc les, permitting them to contract. This expels the sea water and flushe s out the debris from the,out via the mouth. In E. soleae, the myofibr ils associated with the gastrodermis appear to have narrower thick fil aments than nearby parenchymal myofibrils. A similar series of swallow ing movements, involving consecutive contractions of the pharynx follo wed by an egestion event, has been observed in the freshwater temnocep halan ectosymbiont Craspedella sp., from the gills of the Australian c rayfish Cherax quadricarinatus.