fe) THE MOLLUSCA 
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 (Planorbis) or 
may be localised in haematids (red blood corpuscles). These are 
present in Pectunculus, Arca, Ceratisolen |Lankester], Poromya, 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 
