pleurembolic
Capable of being withdrawn by a backward movement of the parts from which it protrudes so that it becomes engulfed.
Some scattered melanophores in fundus of atrium, a few pigment cells along seminal groove. Male atrium (a) narrow in front, wide behind. Its fundus forms a deep fold which receives seminal groove and functions as pleurembolic penis (p).
Where it comes from
From Ancient Greek πλευρά (pleurá, “a rib, a side of something”) + Ancient Greek ἔμβολος (émbolos, “peg, stopper”).
Adapted from the Wiktionary entry pleurembolic (CC BY-SA 4.0).