io THE MOLLUSC A 



The aorta together with the ventricle forms a dorsal vessel com- 

 parable to that of Annelids. Secondarily a second and morpho- 

 logically posterior aorta may be formed, as in the Gastropods and 

 the majority of Lamellibranchs (Fig. 3, E), and even a third (the 

 genital aorta of certain Cephalopods) may be formed in connection 

 with the first. The ramifications of these aortae carry the blood 

 throughout the body. The kidneys, however, are supplied almost 

 entirely with venous blood, from which their cells extract and 

 excrete the waste products of metabolism. Thus the circulation 

 of the kidneys may be described as a portal system. 



The blood is generally a colourless fluid containing amoebocytes 

 and sometimes haematids. It may be of a bluish colour, due to 

 the presence of haemocyanin, an albuminoid containing copper 

 (Fredericq). In other cases it is red, owing to the presence of 

 haemoglobin, which may be in solution in the plasma (Planoi'bis) or 

 may be localised in haematids (red blood corpuscles). These are 

 present in Pectunculus, Area, Ceratisolen [Lankester], Porvriiya, and 

 Neomeniomorpha. The musculature of the buccal bulb has been 

 shown by Lankester in Gastropoda to owe its red colour to 

 haemoglobin impregnating the muscular tissue. The density of 

 the blood is always greater than that of water, greater even 

 than that of sea-water in the case of some marine molluscs. The 

 blood of Anodonta contains ten times more salts than the sur- 

 rounding medium : the density of the blood of Octopus is 1,047. 

 The pressure of the blood in the arteries amounts to from 3 to 5 

 centimetres of water in ^Anodonta and to 108 centimetres of water 

 in Octopus. 



The volume of blood in some groups, particularly in the 

 Lamellibranchs and Gastropods, is so great that it plays a very 

 important part in the turgescence of various parts of the 'integu- 

 ment, by filling the tegumentary sinuses during the relaxation of 

 their muscles. To this end some blood spaces, corresponding to 

 different turgescible organs, are separated by valves which admit of 

 the accumulation of a considerable volume of blood in a definite 

 portion of the body. Examples of this are found in Gastropods 

 and in the valve of Keber in Lamellibranchs. 



Respiration. The venous blood is oxygenated almost exclusively 

 in the superficial tegumentary sinuses of the mantle, this organ 

 receiving a comparatively feeble supply of arterial blood. A 

 portion of the free or ventral surface of the mantle is specialised to 

 form a respiratory organ, through which passes nearly the whole of 

 the blood that is returned to the auricles. These tegumentary 

 organs, enclosing a part of the vascular system, project into the 

 surrounding water in the form of pallial expansions, normally 

 paired, in which the blood is oxygenated. This special part of the 

 circulatory system is often regarded as a separate organ under 



