Scanning electron microscopy (SEM) was used to study the collar region of Deropristis inflata (Molin, 1858), an intestinal digenean of fish, and Echinostoma revolutum (Froelich, 1802), an intestinal digenean of birds. The results showed basic differences in the following morphological features. The collar of D. inflata was open ventrally and dorsally whereas that of E. revolutum was closed dorsally and ventrally, forming a kidney-like ring. The collar of D. inflata was located posterior to the oral sucker, some distance behind the anterior end of the body. That of E. revolutum was located terminally around the oral sucker. The collar of D. inflata had numerous collar spines which varied in size, shape and position. That of E. revolutum had 37 collar-spines, mainly homogenous in their general morphology and with a typical arrangement pattern for 37-collar-spined echinostomes. The collar of D. inflata had few tegumentary papillae whereas that of E. revolutum had abundant papillae. A ventral depressed area occurred in D. inflata, between the acetabulum and oral sucker, dividing the collar into two left and right independent parts. The ventral depression in E. revolutum extended from the ventral sucker to the posterior end of the collar, and was not divided. In D. inflata, tegumentary spines were located on the oral sucker, and the collar was posterior to the sucker. In E. revolutum, the oral sucker and the collar lacked tegumentary spines. The region immediately posterior to the collar also lacked spines in E. revolutum. The collar pattern of D. inflata is undoubtedly more primitive than that of E. revolutum.
General morphology of the oral cavity of the Nile crocodile, Crocodylus niloticus (Laurenti, 1768). I. Palate and gingivae
