MOLLUSCA 



249 



tion into the mouth cavity or into the gullet, and the so-called 

 liver connects with the stomach or intestine. 



291. Respiration. The oxygen may be derived from the 

 water (lamellibranchs, cephalopods, and some gasteropods) 

 or from the air (pulmonate gasteropods). In the latter a 

 pulmonary chamber is formed by the mantle. Blood is richly 

 supplied to the walls of this sac and is there aerated after the 

 manner of lungs. In the water-breathing forms the gills are 



FIG. 112. 



m 



FIG. 112. Diagram showing the heart and general course of the circulation in the Lamelli- 

 branchs. Only a short section is shown, a, auricle (right), with slit to ventricle; b, the body 

 (region of spaces, lacunae, capillaries); g, the region of the gills, with capillaries; k, kidneys, with 

 their capillaries; m, the mantle and capillaries; v, the ventricle from which arteries pass forward and 

 backward; v.c., "vena cava," in which the blood collects on returning from the tissues of the body. 



Questions on the figure. Follow by the arrows and letters the general course of 

 the blood flow. How many sets of capillaries are passed by the blood which goes 

 to the mantle? By that which goes to the system, before returning to the heart? 

 What changes take place in the blood in the capillaries of the various regions? 



variously constructed. Lamellibranchs possess a pair of "gill- 

 plates" hanging in the mantle cavity on either side the body. 

 These are made up of an immense number of ciliated tubular 

 filaments which intercommunicate in a complicated lattice- 

 work. To the naked eye they appear as thin sheets with stria- 



