They may be characterized as soft, cold-blooded animals, without distinctly marked external division into segments (as in Worms); their cerebral ganglia (the centre of the nervous system) lie above the commencement of the gullet, and are connected with the inferior ganglia by nerve-chords. Their heart consists of two or more chambers, and is situated on the dorsal side of the animal; it drives the blood into spaces between the various organs of the body. Only the Cephalopods possess internal cartilages, but all are without a bony internal skeleton; in the majority this is compensated by an external hardened shell which is formed (secreted) by the outer covering of the animal termed the _mantle_. [Sidenote: The shell.] The shell may consist of two parts (valves), as in the Oyster, or may be single, as in the Whelk and Limpet, or composed of a series of plates, as in the “Coat-of-mail” shells or _Chitons_: when well developed it is hardened by a rich deposit of carbonate of lime; but it may be gelatinous, as in _Cymbulia_, or altogether absent, as in _Octopus_; it may cover and protect the body, as in the Oyster, lie within the folds of the mantle, as in the Sea-hares (_Aplysiidæ_), or it may be quite internal, as in the horny “pen” of the Squid. It may be elongated, as in the Elephant Tooth-shell (_Dentalium_), cup-shaped, as in the Limpet, or spirally coiled, as in the Snail. [Sidenote: Description of the animal.] The mantle may form a free fold on either side of the body, as in the Bivalves, or it may become largely attached to the body-wall, as in the Snail or the Slug, and so give rise to an air-chamber, which, when its walls are richly supplied with blood, serves as a lung. The ventral surface of Molluscs is produced into the so-called “foot,” which may be very variously modified. The foot may be more or less hatchet-shaped, or curved and capable of serving as a leaping-organ, or sole-shaped and adapted for creeping; its margins may be produced into elongated processes, as the so-called arms of the Octopus, eight in number and provided with suckers, or of the Nautilus, where the arms are much more numerous, but shorter and without suckers. In the Cephalopods, also, another part of the foot may fold over from either side and form a median funnel, through which the water of respiration is driven outwards, causing the animal to move in the opposite direction—this part of the foot having, therefore, still the function of an organ of locomotion. By means of their muscular foot the _Solenidæ_, or Razor-shells, burrow in the sand, the Pond-Snails (_Limnæidæ_) crawl on aquatic plants and swim reversed on the surface of the water, the Limpet clings to the rock, and the Cockles and _Trigonias_ take surprising leaps. [Sidenote: The operculum.]