MOLLUSCA. 



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 

 striations passing from the dorsal to the ventral margin (see 

 dissection of clam). The walls of the gills contain blood 

 vessels, and the water, assisted by the action of the cilia, circu- 

 lates over and through the gills. In the cephalopods and 

 aquatic gasteropods the gills occur as tufts of filaments, which 

 may or may not be covered by the mantle. In addition to 

 these special organs the mantle and the soft body surface 



FIG. no. Diagram showing the heart and general course of the circulation in the 

 Lamellibranchs. Only a short section is shown, a, auricle (right), with slit to ven- 

 tricle; 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, be- 

 fore returning to the heart? What changes take place in the blood in the 

 capillaries of the various regions? 



