Photo: Harold Bastin.
The group of the Mollusca includes such common forms as cuttle-fishes, whelks, slugs, snails, mussels, and oysters. These, it will be observed, comprise marine, freshwater, and land forms. The molluscs, like the next two groups with which we have to deal, have made a conquest of the land, though in the present instance it cannot be regarded as very complete. The anatomy of the group shows much variation, and only a few of the leading features can be alluded to. The digestive system is highly developed. The mouth is provided with a jaw or jaws, and with a tongue-like ribbon, which is covered with rows of teeth, like a file, and by whose action the food is torn and disintegrated. A gullet leads from the mouth to a stomach, which is followed by an intestine. Salivary glands and a large hepatic gland or liver are present. Respiration occurs partly through the skin, but special organs also exist for this function, gills in the water forms, and a lung cavity in those which breathe air. There is a well-developed blood system, and generally a heart; the blood is pumped direct from the heart to the general body tissues, and returns to it by way of the kidneys or nephridia, which purify it of waste materials, and the respiratory organs, where it is freed of carbon dioxide and supplied with oxygen. The nervous system varies greatly, but a pair of cerebral ganglia—a brain—is usually present. There is a particularly keen sense of smell, and taste and hearing may also easily be shown to exist. Some forms are blind, from which condition there is a regular series of stages of development of the eye, up to forms in which it becomes a highly perfected organ, with cornea, iris, lens, and retina. The close similarity between this and the ordinary vertebrate eye, which must have evolved quite separately, is one of the strangest coincidences of Evolution. Thus in many ways the molluscs are to be regarded as highly specialised types. But in two important directions, in intelligence and in their arrangements for locomotion, they stand as a group on a low plane of development. Figs. 49, 50, and 51 illustrate some of the forms met with in the group. The origin of the molluscs, as well as that of the Echinoderms, is wrapped in obscurity. That each group is derived from some form of worm is probable, yet some zoologists hold even such a general statement as this to be lacking in support.
Fig. 48.—Sea Urchin.
1, With spines broken off; 2, with spines on.
Fig. 49.—Molluscs—Univalves.
Fig. 50.—Molluscs—Bivalves.
