140 



BOYS AND GIRLS IN BIOLOGY. 



you found it on the lattice-work of the mussel's gill- 

 pockets. As the screw scoops the water through the 

 branchial or GiLL-CHAMBEE, the blood takes out the 

 oxygen from the air in the w^ater and gives back car- 

 bonic acid. You remember how the strong hairs (ci- 

 lia) of the pockets sweep the water along over the 

 mussel's gills, and how the little blood-vessels take up 

 their oxygen and give up their carbonic acid. The 

 gills that are fastened to the legs, or limbs, move when 

 the limbs move, and the faster they go the more water 

 they use, just as the faster boys run the more air they 

 must have ; so the more pure water for the lobsters 

 and the more pure air for the boys, the purer and the 

 better the blood for them both. So much for the lob- 

 ster's breathing, or respiration. Next we will study 

 his circulation. His blood must move, else it would 

 not get in and out of the gill-plumes. Beneath the 

 head-shield (carapace) and behind the seam which 

 joins the shells of the head and breast, you will find 

 another chamber ; it lies just behind the stomach (Fig. 

 116), and between the gill or branchial chambers. 

 This is the heart-sac, or pericardium. You remember 

 you found one in the mussel. In the heart-sac, or peri- 

 cardium, lies a six-sided sac (Fig. 122), and this is the 

 heart. It has but one room — no auricles like the mus- 

 sel's — nothing but a ventricle, which is slung in the 



