A single aorta occurs only in the Amphineura and in the Tetrabranchiate Cephalopoda. In all the other groups there are two aortae, leading out of the anterior and posterior ends of the ventricle in Pelecypoda and Dibranchiate Cephalopoda, while a single aorta leads out of the posterior end alone, and subsequently bifurcates, in most of the Gasteropoda. One aorta, the cephalic, supplies the front part of the body, the oesophagus, stomach, mantle, etc.; the other, the visceral aorta, supplies the posterior part, the liver and sexual organs. The general circulatory system in the Mollusca has not yet been thoroughly investigated. As a general rule, the blood driven from the ventricle through the aorta into the arteries, passes, on reaching the alimentary canal and other adjacent organs, into a number of irregular spaces called _lacunae_. These in their turn branch into _sinuses_, or narrow tubes covered with muscular tissue, which penetrate the body in every direction. In the Dibranchiate Cephalopoda true capillaries are said to occur, which in some cases form a direct communication between the arteries and veins. According to some authorities[277] capillaries and veins exist in certain Pelecypoda in connexion with the intestinal lacunae, but this again is regarded by others as not established. A similar difference of opinion occurs with regard to the precise function of the foot-pore which occurs in many Mollusca, some holding that it serves as a means for the introduction of water into the blood-vascular system, while others regard it as a form of secretion gland, the original purpose of which has perhaps become lost. =Blood.=--As a rule, the blood of the Mollusca--_i.e._ not the corpuscles but the liquor sanguinis--is colourless, or slightly tinged with blue on exposure to the air. This is due to the presence of a pigment termed _haemocyanin_, in which are found traces of copper and iron, the former predominating. _Haemoglobin_, the colouring matter of the blood in Vertebrates, is, according to Lankester,[278] of very restricted occurrence. It is found--(1) in special corpuscles in the blood of _Solen legumen_ (and _Arca Noae_); (2) in the general blood system of _Planorbis_; (3) in the muscles of the pharynx and jaws of certain Gasteropoda, _e.g._ _Limnaea_, _Paludina_, _Littorina_, _Chiton_, _Aplysia_. This distribution of haemoglobin is explained by Lankester in reference to its chemical activity; whenever increased facilities for oxidisation are required, then it may be present to do the work. The Mollusca, being as a rule otiose, do not possess it generally diffused in the blood, as do the Vertebrata. The actively burrowing _Solen_ possesses it, and perhaps its presence in _Planorbis_ is to be explained from its respiring the air of stagnant marshes. Its occurrence in the pharyngeal muscles and jaws of other genera may be due to the constant state of activity in which these organs are kept.[279]