JUDGED purely by external appearances one would hardly suspect anything in the nature of a close relationship between a limpet and a cuttlefish. The one small, inoffensive, appearing hardly ever to move, its body completely enveloped in a hard shell; and the other large, active, belligerent, with a naked body differing markedly in general features from that enclosed within the limpet’s shell. Yet these two and many more creatures almost equally diverse in form are grouped together under one heading : the Mollusca. For general purposes it is convenient to describe the molluscs as animals with soft bodies usually enclosed in one or more shells,
but this, like all rules, has many exceptions. That it is applicable to the limpet is quite obvious, but to find the shell of a cuttlefish it is necessary to cut the beast open. In fact, the familiar cuttlebone corresponds to the shells of other molluscs.
The three main groups of the Mollusca are the Cephalopoda, including the octopus, squid and nautilus; the Gastropoda, or univalves, including snails, whelks and limpets, which move about on a broad muscular expansion of the body, known as the foot, and have a single shell usually spirally coiled; and the Lamellibranchiata, or bivalves, including oysters, mussels, cockles and others of their kind, with the body usually enclosed within a shell formed of two parts, or valves, and hinged along one side.
The last two groups are well known to us in a general way, even to those who, zoologically speaking, are the least initiated and none will fail to recognise in them creatures of a sluggish habit. In fact, so characteristic is their slow rate of locomotion that the very word sluggish is derived from the name of a shell-less Gastropod mollusc. And has not the snail’s pace at all times been regarded as the epitome of the slowest rate of locomotion?