Comparative anatomy of slime glands in onychophora (velvet worms)

Onychophorans use a unique hunting and defense strategy, which involves the ejection of an adhesive slime secretion produced by a pair of specialized glands. So far, a comparative study on the anatomy of these glands has not been carried out among different species. In this article, we compare anatomical features of slime glands in representatives of two major onychophoran subgroups, the Peripatopsidae and the Peripatidae, from different parts of the world. Our data show that the musculature of the reservoir is conserved whereas the composition of the secretory duct displays taxon-specific variation. Major differences concern the arrangement of glandular endpieces, which are distributed along the duct in Peripatopsidae but condensed in numerous repeated rosettes in Peripatidae. In addition, there are differences in the attachment pattern of slime glands to the inner surface of the body wall and to the outer surface of the gut between the two major onychophoran subgroups. A tube-like structure with a putative valve-like function is found at the transition of the secretory duct and the reservoir in the five Peripatopsidae species studied whereas it is absent in the two representatives of Peripatidae. Our findings suggest that the arrangement of musculature in the reservoir of the slime gland has remained unchanged since the divergence of Peripatidae and Peripatopsidae, while the composition of the secretory duct has been altered in one of these groups. However, the direction of evolutionary changes in duct composition cannot be determined unambiguously due to current uncertainty regarding the phylogenetic relationships of Onychophora.