view (held on other grounds) that the Polyclads are the most primitive of the Turbellaria, is soundly based.
The grounds for this view are the radial symmetry of several Polyclads and the supposed origin of gonads and excretory flame-cells from the walls of gut, the occurrence of nematocysts in Anonymus, one of the most radially constructed Polyclads, and lastly the presence of two peculiar animals Ctenoplana and Coeloplana, which suggests a transition from Ctenophora to Polyclads.
The argument has come to rest on the agreement between the cell-lineage of Polyclads and that of certain Mollusca and Annelids.
This resemblance is considered by Hubrecht (5) to give reason for concluding that the Polyclads are an offshoot, and possibly a degenerate offshoot, from the early Coelomate stock.
From such beginnings the evolution of the Turbellaria leads first through the Acoelous forms in which the central syncytium is partly differentiated into digestive, muscular and skeletotrophic tissue, then to the more specialized Rhabdocoela, and so through the Alloeocoela to the Triclads and finally to the Polyclads.
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