These snails have learned to live with a reduced shell allowing more energy into survival rather than shell development. Shell reduction in pulmonates can occur in several different ways. Two important evolutionary pathways are:
(1) a decrease in shell sculpture and thinning of the shell; and
(2) a decrease in the number of whorls and corresponding increase in the size of the
Since shell reduction occurs gradually over the course of many generations, there are many species that have body forms intermediate between snails and slugs. These are referred to as semi-slugs. A semi-slug still has an external shell, but cannot fully retract into it. Its stomach and digestive gland remain in the visceral hump, but the crop is at least partially contained in the foot cavity [adapted from Hyman, 2005].
This means that desiccation is a major issue and helicarionids have learned to adapt their lifestyle to minimise this.
There are 2 types of helicarionids:
- Glass-snails: This is a group of semi-slugs and one of the most speciose families.
Chillagoe Glass-snail, Einasleighana antiqua
Gayndah Glass-snail, Delinitesta gayndahensis
- Semi-slugs: Semi-slugs are snails that cannot retract into their shell as it is much reduced in size.
L to R: Thularion semoni, Mt Sorrow, Queensland. J. Stanisic; Mysticarion porrectus, Point Lookout, NE New South Wales. Photo: Fran Guard; Macularion albimaculosa, Lamington, South Queensland. Photo: IBISCA; Parmavitrina megastoma, east of Tenterfield, eastern, New South Wales. Photo: Jan White.