LOCOMOTION IN THE PRIMITIVE PULMONATE SNAIL MELAMPUS BIDENTATUS: FOOT STRUCTURE AND FUNCTION

¹ Department of Zoology, Washington State University, Pullman, Washington 99164, and Marine Biological Laboratory, Woods Hole, Massachusetts 02543

The foot of Melampus is subdivided into an anterior propodium and a posterior metapodium by a permanent transverse groove. Locomotion in Melampus consists of repetition of a cycle of events that pass from posterior to anterior; this cycle has been named a crawl-step. Three stages in the crawl-step have been identified: Metapodial shortening is produced by the action of columellar muscles and this action forces blood anteriorly to extend the propodium. Metapodial lengthening is produced by muscle action within the metapodium and extends the metapodial region forward at the expense of the propodium. Propodial elevation is produced by columellar muscles and prepares the propodium to "step" forward while fluid invasion occurs in the first stage.