The para-receptacle structure (PRS) was first described in Neoechinorhynchus (Neoechinorhynchus) qatarensis Amin, Saoud, and Alkuwari, 2002 (Neoechinorhynchidae), as a prominent vesicle connected proximally to the anterior body wall and distally to the posterior wall of the proboscis receptacle then extending into the body cavity of males and females as a posterior conduit tube. The PRS has since been observed in Neoechinorhynchus (Neoechinorhynchus) golvani Salgado-Maldonado, 1978 (a new subgeneric diagnosis), and described in a member of another eoacanthocephalan family, Acanthogyrus (Acanthosentis) parareceptaclis Amin, 2005 (Quadrigyridae). In the latter species, a similar structure was also observed at the posterior end of the trunk. Our optical and transmission electron microscopy (TEM) observations in specimens of N. qatarensis show the PRS as a tandemly arranged set of nucleated cells delimited by a double-walled membrane and a muscular wall. It appears to be contractile with a secretory function related to changes in the hydrostatic pressure that may be associated with the extrusion and retraction of the proboscis in worms with weakly developed proboscis receptacle musculature. This is the first detailed study of the PRS and of its proposed function in the only 3 acanthocephalan species from which it has been identified to date. It is considered as an archaic structure present only in a few representatives of some primitive taxa having only a single-walled proboscis receptacle wall.
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