Snails, Limpets and Mussels: Mollusca

Abstract

Besides several small, marine groups, the phylum of molluscs (Mollusca) contains three large, well-known classes, that of the gastropods (snails and slugs, Gastropoda) (Barnes 1980; Brown and Lydeard 2010; Burch 1989; Damborenea et al. 2019; Falkner et al. 2001; Fehér et al. 2006; Fitter and Manuel 1986; Illies 1978; Kriska and Tittizer 2009; Quigley 1977; Schwab 1999; Strong et al. 2008; Thorp and Rogers 2015), bivalves (mussels and clams, Bivalvia) (Barnes 1980; Damborenea et al. 2019; Fitter and Manuel 1986; Illies 1978; Kriska and Tittizer 2009; Pechenik 1985; Quigley 1977; Schwab 1999; Thorp and Rogers 2015) and cephalopods (Cephalopoda). Among them, gastropods and bivalves occur in freshwater habitats. Gastropods, like all molluscs, are unsegmented animals with a body consisting of head, foot, mantle and visceral hump. The head of aquatic snails bears a pair of tentacles. At the base of each tentacle is a pit eye capable of sensing the direction of light. The mouth opens on the ventral side of their head. Their characteristic feeding organs are the radula and a chitinised jaw both used for rasping or scraping up the food. The inner organs of the molluscs are within the visceral hump, which is surrounded by a fold of the body wall called mantle. The function of the mantle is to secrete the calcareous material composing the shell of the animal. The shell of snails is composed of one single piece that may be either coiled or cap-like. The tip of the coiled shell is the apex. The whorls expand continuously downwards, right till the opening called aperture at the bottom. The contact lines of the adjacent whorls are the sutures, visible on the surface of the shell. Gilled aquatic snails close the aperture with a lid, the operculum, when they retract their body into the shell. Pulmonate snails lack such an operculum. In the central axis of the shell a columella is formed by the tightly coiled whorls. If the coils are rather loose, a hollow tube is visible in the axis instead, that has a small opening at the bottom, called umbilicus. The shell may be either dextral or sinistral. Dextral shells are coiled clockwise in top view, i.e. in frontal view their aperture is situated to the right of the vertical midline of the shell. The respiratory organ is either a gill within the mantle cavity or a special, vasculated region of the mantle wall, the lung, which is capable of absorbing atmospheric oxygen.