Evolutionary changes in the integument of the onychophoranPlicatoperipatus jamaicensis(Peripatidae)

Abstract

AbstractThe dorsal integument of nearly all species of Peripatidae—one of the two major subgroups of Onychophora (velvet worms)—typically exhibits 12 plicae (=annuli) per segment. The only exception might occur inPlicatoperipatus jamaicensis, from which 24 putative plicae per segment have been reported. Hence, the number of plicae might have been duplicated in this species. If so, one would expect that the structure of the duplicated plicae would resemble that of all other onychophoran species, and that the structures commonly associated with the plicae, including crater‐shaped papillae and hyaline organs, would also have been duplicated. To clarify whether there was indeed such duplication, we compared the structure of the integument in embryos and adults ofPl. jamaicensis(with the putative number of 24 plicae per segment) andPrincipapillatus hitoyensis(with the common number of 12 plicae per segment). Our scanning electron microscopic data revealed that embryos of both species have 12 plicae per segment. While this number persists in adults ofPr. hitoyensis, 12 additional rows of papillae (=pseudoplicae) occur in the dorsal integument in adults ofPl. jamaicensis. These pseudoplicae differ from the typical plicae of other onychophorans in that they are not equipped with primary papillae and are situated in furrows between the true plicae, as evidenced by the position of hyaline organs and crater‐shaped papillae. Our data further show that the number of the ventrolateral plicae, crater‐shaped papillae, and hyaline organs is similar in the two species studied. This suggests that the plicae have not been duplicated in the integument ofPl. jamaicensis, but rather that additional pseudoplicae have been inserted between the 12 original plicae. These pseudoplicae might have led to a denser package of dermal papillae in the dorsal integument ofPl. jamaicensis, but the functional significance of this evolutionary change is unknown.