CIRCULATORY SYSTEM — BLOOD AND ITS CONSTITUENTS. 295 
elimination from the body. Analysis shows Carbonate and Phosphate 
of Lime, Carbonate and Chlorhydrate of Soda, O.xide of Iron, and 
a trace of O.xide of Manganese to be some of the constitnents of 
mollnscau blood. 
The blood may form fnlly half the total weight of the body in the 
Pelecypoda and about one-sixth in the Pnhnonates, this relatively 
large volume enabling a portion to be utilized to protrude or stiffen 
different parts or organs of the body, as strikingly seen in the pro- 
trusion and e.xtension of the 
tentacles in the Gastropoda and 
in the tnrgescence of the foot in 
the Pelecypoda, this being ac- 
complished by permitting the 
inrush of blood to the j)articnlar 
organs to continue and retaining 
it therein by the action of suit- 
ably dis})Osed sphincter muscles, 
the most important being known 
as “ Keber’s Valvule,” whose 
action permits the free ingress 
of blood to the foot, but by the 
closure of the gveat afferent 
renal vein prevents its esca})e therefrom, thus retaining all the blood 
within the foot and enabling it to (piickly become firm and rigid for 
locomotory purposes. 
H.emocyanin (atp.a, hlood ; Kvaveos, dark bine), a proteid in 
combination with copper, with a chemical composition of Cgr, 7, 
Hi 30 3 ^22:5 CnS.i 0.5 8, is also usually present in mollnscan blood 
and functions in a similar, but feebler, way to luemoglobin, by 
absorbing oxygen from the air or water and carrying it to the more 
remote tissues of the body, the bluish or violet colour, characterizing 
it when rich in oxygen, being gradually lost as that substance becomes 
abstracted by the tissues with which it comes in contact. 
H.EMOGLOBIN (af/xtt, blood ; ghhus, a sphere), is an albuminoid in 
association with iron, which during respiration enters freely into an un- 
stable combination with oxygen, from which it is again separated by 
the tissues as the blood circulates through the body. It is present in 
the buccal muscles of many species, whose energetic action calls 
for more active o.xygeuation than hremocyanin can accomplish. The 
Fig. 5S6. — Keber’s valvule in Anodonta cy^nca 
var. celU'usis C. Pfr., greatly enlarged (after 
Fleischniann). 
k.v, sphincter of Keber’s valvule ; r.c. renal 
cavities ; pc. pericardium ; r.p.f, reno-pericardial 
funnel. 
