These folds are called the genital pleurae because they contain the bulk of the gonads.
Correlated with the presence of the genital pleurae there is a pair of vascular folds of the basement membrane proceeding from the dorsal wall of the gut in the postbranchial portion of the branchio-genital region, and from the dorsal angles made by the pleural folds with the body-wall in the pharyngeal region; they pass, in their most fully developed condition, to the free border of the genital pleurae.
Since there are many species which do not possess these genital pleurae, the question arises as to whether their presence or their absence is the more primitive condition.
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