Montana Field Guide

A small to medium-sized slug, reaching 25 to 40 mm extended. Dorsal base color is gray, sometimes lighter on the sides, tentacles dark. The mantle is granulose, about 1/3 the body length, and the same color as the back, marked with numerous small dark spots bounded by a pair of brownish lateral bands close to the mantle margin. The pneumostome is in the anterior half of the mantle on the right side, posterior to the mantle cleft. The tail has a pair of brownish or darker gray lateral bands extending to near the posterior end, lacks a mid-dorsal keel, and is covered dorsally with many fine tubercles in a series of rows. The sole is very pale, undivided (lacking two longitudinal furrows), the mucous clear and not sticky (Hendricks 2012, Burke 2013). Internal anatomy is described by Pilsbry (1948) and Forsyth (2004).

Arion lack an elevated visceral hump, possess a mantle about 1/3 the length of the animal, have an undivided foot and lack a dorsal keel on the tail, have a relatively broad tail with a caudal mucous pit at the tip where the pedal furrows meet, and a pneumostome in the anterior half of the mantle. A. circumscriptus differs from other members of the genus by a combination of it's small to medium size, dark gray to brown color (paler on sides) with thin dark line on sides of mantle and tail, clear mucous, and presence of small black speckles on the mantle.

Occupies moist to wet sites (stream-sides, marshy areas) within forested or shrubby locations, usually near areas of human activity, such as campgrounds, gardens and other disturbed sites. Canopy species include western redcedar, Engelmann spruce, western hemlock, grand fir, black cottonwood, aspen, mountain ash, and water birch, secondary canopy species include alder and mountain maple. Most often found under woody debris and rocks (Hendricks 2012).