Orders: Heterocotylea, Aspidocotylea, Malacocotylea.
The Trematodes are divided into three orders, primarily distinguished by the character of their suckers, viz.: Heterocotylea, Aspidocotylea and Malacocotylea.
This structure is regarded as the homologue of a canal (Laurer's canal) which in the Heterocotylea opens into the (After Monticelli.
Others again consider that the whole cycle is a metamorphosis which, beginning in the Heterocotylea as a direct development, has become complicated in the Holostomidae by a larval history, and finally in the Malacocotylea has acquired additional complexity by the intercalation of two larval forms, and is thus spread over several generations.