36 PEARLS [CH. 



obtain their food. They searcli not, neither do they 

 place food in their mouths by special appendages. 



Reference has been made once or twice to the 

 gills. It must be said here that they are not the only 

 organs concerned in respiration. 



The mantle lobes themselves are very efficient 

 respiratory organs, for they contain numerous blood 

 spaces, and the wall separating this blood from the 

 sea-water in the cavity of the shell is extremely thin 

 and allows the interchange of the gases. 



The Blood System. 



All bivalves possess a heart and system of blood 

 vessels. This perhaps is not common knowledge, 

 because the blood of these molluscs is practically 

 alw ays colourless. The absence of the characteristic 

 red haemoglobin of human 'blood does not render the 

 blood different so far as function is concerned. There 

 are other chemical compounds present instead, which 

 help to carry out chemical changes similar to those 

 performed by haemoglobin in human respiration. 



The heart consists of one ventricle, a central, 

 dorsally situated, muscular-walled bag, and two 

 auricles, one on either side, with much thinner walls. 

 The ventricle contracts or " beats " rhythmically, and 

 at each contraction the contained blood is forced 

 out through two arteries (not shown in fig. 4) to the 



