ANATOMY AND PHYSIOLOGY OF (JRUSTAOEA 123 



as great a surface as possible for the passage of these gases, and 

 they must be protected from injury, such as being bitten off 

 by fishes. Consecjuently we find the gills of the lobster tufted 

 and hidden under a fold of the skin — the carapace. From 



Fig. 124. — Respiratory organs of Astacus fluiaatilis. The gill cover removed, 

 and gills undisturbed, ai, small antenna, ao, large antenna ; cihi, ab-i, first 

 and second abdominal segment; ep^, " scaphognathite " ; plb, " pleuro- 

 branch"; pdfer, 12, " podobranchs " ; jsij, first pleopod ; 6-13, thoracic ap- 

 pendages. From Lang, " Comparative Anatomy." 



or near the base of each leg one to four gills arise and lie in the 

 gill chamber. The water is pumped through this gill chamber 

 by the motion of certain fanlike organs. Each gill contains 

 a vessel carrying blood from the body to the gill (afferent) 

 and one from the gills to the body (efferent). The total 

 number of gills (Gk. hranchia) varies with the species (Fig. 124). 

 Usually one or two spring from the foot itself (podobranch) ; 

 one or two from the joint (arthron) connecting foot to trunk 

 (arthro-branch) ; and one from the side wall (pleuron) of the 

 body (pleurobranch'). 



I Get a lobster or crayfish and see how many gills of each of these classes 

 occur on each segment of the thorax (Fig. 125). 



