Orders: Nemertini, Turbellarii, Trematodes and Hirudinei.
The second point of difference between tapeworms and Trematodes lies in the absence of a definitely demonstrable " brain."
The concentration of nervous matter and ganglionic substance at the oral end of Trematodes is equivalent to the " brain " of the Planarians, but the similar thickening in the scolex of Cestodes is by no means so certainly to be called by that name.
Those Cestodes which possess no very distinct organ of attachment (such, for example, as Gyrocotyle) have no distinct ganglionic thickening more pronounced at one end of the body than at the other; and as these are forms which have retained more primitive features than the rest, and show closer affinity to the Trematodes, it seems highly probable that the complicated nervous thickening found in the scolex, and often compared with the " brain " of other Platyelmia, is a structure sui generis developed within the limits of the sub-class.
The vagina of Cestodes is undoubtedly comparable with the so-called " uterus " of Trematodes, but the nature of the Cestode uterus is not so clear.
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