Crustacea. 



6 7 



There are Two Sets of Gills. One row is attached to 

 the bases of the thoracic appendages ; these are the foot 

 gills. A second row arises from the joints by which the 



FIG. 48. CROSS SECTION OF A CRAYFISH THROUGH THE HEART. 



Showing the gills, blood currents, and water currents. 



thoracic appendages are attached to the thorax; these 

 are the joint gills. (In the lobster there is a third set, 



