PHYLUM MOLLUSCA 



287 



of vertical gill filaments (Fig. 176), strength- 

 ened by chitinous rods and connected to 

 one another by horizontal bars. Cross or 

 interlamellar partitions between the two 

 lamellae divide the gill into many vertical 

 water tubes. Dorsally, the water tubes of 

 each gill join a common suprabranchial 

 chamber. The gills function in respiration by 

 water circulating through their interiors. It 



enters through the incurrent siphon and 

 flows over the gills. On the surfaces of the 

 gills there are many microscopic openings, 

 water pores, through which the water is 

 driven by ciliary action into the water tubes. 

 The water then passes dorsally in the water 

 tubes to the suprabranchial chambers and 

 along them to the excurrent siphon. Ex- 

 change of respiratory gases takes place 



jprabranchial chamber 

 ■Excurrent siphon 



•Incurrent siphon 



Gill filament 



Interlamellar partition 



tjkT' Afferent vein 



— Efferent vein 



: — Water pore 



T■^v:^.^^::^> 

 I ir - -r ^ 



Water pores 

 Chitinous rod 



Water tube 



Figure 176. Respiratory system of a fresh-water clam. Left, a whole gill. Right, horizontal 

 section through a gill showing arrangement of gill filaments, blood vessels, and water tubes. 

 Arrow shows direction of water currents. 



through the walls of blood spaces located 

 in the partitions of the water tubes. 



The two kidneys (nephridia) lie just be- 

 neath the pericardial cavity. Each nephrid- 

 ium is folded upon itself and differentiated 

 into glandular (dark spongy mass) and 

 bladderlike portions. The structure and re- 

 lationships are such that one end of the 

 nephridium opens into the pericardial cavity 

 and the other end (external) opens into a 

 suprabranchial chamber. Liquid wastes 



within the pericardial cavity may enter the 

 tubule; or metabolic wastes carried by 

 the circulating blood may be removed by 

 the cells in the glandular portion of the 

 nephridium. 



Nervous system and sense organs 



Three pairs of ganglia are present (Fig. 

 174): cerebropleural ganglia, pedal ganglia, 

 and visceral ganglia. The sensor}' structures 

 include light receptors in the siphon mar- 



