They have similar worldwide distribution patterns.
The members of the slug family Philomycidae are found in eastern North America and southeast Asia. Here is their distribution range.
Verbreitungskarte der Philomyciden, the distribution map of the Philomycidae from Hoffmann (1924). Keep in mind that the taxonomy and the known ranges of the philomycid species may have since changed, but the overall distribution is probably still the same.
And here is the distribution range of the genus Toxicodendron.
From The Poisoned Weed by Donald G. Crosby (2004).
Toxicodendron radicans is the poison ivy. There are several subspecies of T. radicans in eastern North America and 2 subspecies in China, Taiwan and the Kurile Islands.
Is it a coincidence that a family of animals and a species of plants have roughly the same distribution ranges each characterized by 2 widely separated areas?
This peculiarity is not limited to the Philomycidae and Toxicodendron radicans. There are some other groups of animals and plants with similar shared ranges in eastern North America and Southwest Asia.
*Hoffmann, H. 1924. Zur Anatomie und Systematik der Philomyciden. Jenaische Zeitschrift für Naturwissenschaft 60: 364-396.