On the one hand the Turbellaria are considered to be an offshoot of the early Coelomate stock, on the other they are held to be descendants of a simpler two-layered stock.
This segregation of the germ cells and their independence of the intestinal sac is an indication that the origin of these cells is not coelomic nor enteric, and until we possess further information as to the evolution of the complex genitalia of the higher Turbellaria we cannot hope to understand the presence of such highly modified structures in animals of an otherwise low grade or organization.
The class as a whole is linked to the Turbellaria not only by its similarity of structure, but by the intermediation of the singular class the Temnocephaloidea (see Planarians), which in habit and in organization form an almost ideal annectant group.
Pharynx, and he sums up their relationship to the Annelids by thestatement that to a certain extent the Nemertines represent Turbellaria which in the course of time have copied certain features of an Annelid character.
In effect (6) it traces the Turbellaria to small two-layered organisms consisting of an outer ciliated epidermis and a central syncytial tissue.
Of Turbellaria and Ne- a, anus; ap, apical organ, correspond mertina.
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.
A single or paired accessory gonopore is met with in many Trematodes just as in certain Turbellaria (e.g.