The development of these organs, which in the Protonemertine are but grooves in the epidermis, not far removed from the similar cephalic slits of many Turbellaria, reaches its height in Drepanophorus.
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.
Whether this view be adopted or not, and whether the Turbellaria be regarded as nearly related or only remotely connected, there can be little doubt that the Nemertines resemble the Turbellaria more nearly than they do any other group of animals.
The chief peculiarities that distinguish Trematodes from their free-living allies, the Turbellaria, are the development of adhering organs for attachment to the tissues of the host; the replacement of the primitively ciliated epidermis by a thick cuticular layer and deeply sunk cells to ensure protection against the solvent action of the host; and (in one large order) a prolonged and peculiar life-history.
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.
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