Studies on metacercarial excystment in Himasthla leptosoma (Trematoda: Echinostomatidae) and newly emerged metacercariae

Metacercarial cysts of Himasthla leptosoma were subjected to a solution containing bile salts, trypsin and l cysteine at 41°C. The treatment induced intense metacercarial activity and after 20 min metacercariae burst through the cyst walls and emerged. Electron microscopy demonstrated that organisms burst through a small area of cyst wall which was devoid of a layer of lamellae present elsewhere towards the innermost surface. The appearance of ruptured cyst walls indicated that they had been softened by the excystment medium. Newly emerged metacercariae possessed a reniform collar of 29 cephalic spines and these were sometimes withdrawn into pits, presumably by action of a muscle complex in the head region. Sensory papillae were distributed in a bilaterally symmetrical arrangement around the anterior sucker and none were visible on the surface of the ventral sucker. Tegumental spines were found only from a point some distance behind the head collar to the region of the ventral sucker. The most anterior spines were simple and peg-like and they quickly merged posteriorly into more complex palmate forms.