The Trematodes are somewhat modified in accordance with their ectoor endoparasitic life, but they exhibit such a close similarity of structure with the Turbellaria that their origin from Planarians can hardly be doubted, and indeed the Temnocephaloidea (see Planarians) form an almost ideal annectant group linking the ectoparasitic Trematodes and Rhabdocoel Planarians.
The excretory pore is terminal and posterior in endoparasitic forms: paired, anterior and dorsal in the ectoparasitic class.
In most endoparasitic Trematodes the accessory gonopore is a median and dorsal structure.
If ectoparasitic and attached to the skin, apertures or gills, the Trematode adopts more elaborate adhesive organs and undergoes a less complex development than are required for the endoparasitic members of the class.
A, in endoparasitic Trematodes (Malacotylea).
In the majority of endoparasitic forms it is merely a muscular disk just behind the mouth; but in the Aspidocotylea this sucker forms a muscular ribbed sole extending over the greater part of the ventral surface (fig.
It is usual to consider the ectoparasitic habit as leading up to the endoparasitic one.
TAPEWORMS. The Cestodes or Tapeworms form a class of purely endoparasitic Platyelmia, characterized by their elongate shape, segmented bodies, and the absence of a digestive system.
In the endoparasitic trematodes the uterus is the only passage by which fertilization can be effected, and in cases of cross and selfimpregnation this duct is physiologically a vagina.