MOLLUSKS 



IOI 



chamber for ihe gills is made by the joining of the mantle 

 flaps below, along the ventral line. The mantle edges are 

 separated at two places, leaving openings called exhalent 

 and inhalent sip/ions. 



Fresh water with its oxygen, propelled by cilia at the 

 opening and on the gills, enters through the lower or 

 inhalent siphon, passes between the gills, and goes to an 

 upper passage, leaving the gill chamber by a slit which 

 separates the gills from the foot. 

 For this passage, see arrow 

 (Fig. 194). The movement of 

 the water is opposite to the way 

 the arrow points. After going 

 upward and backward, the water 

 emerges by the exhalent siphon. 

 The gills originally consisted of 

 a great number of filaments. 

 These are now united, but not 

 completely so, and the gills still 

 have a perforated or lattice 

 structure. Thus they present a 

 large surface for absorbing oxy- 

 gen from the water. 



The mouth is in front of the foot, between it and the 

 anterior adductor muscle (Fig. 194). On each side of the 

 mouth are the labial palps, which are lateral lips (Fig. 195). 

 They have cilia which convey the food to the mouth after 

 the inhalent siphon has sent food beyond the gill chamber 

 and near to the mouth. Thus both food and oxygen enter 

 at the inhalent siphon. The foot is in the position of a 

 lower lip, and if regarded as a greatly extended lower lip, 

 the animal may be said to have what is to us the absurd 

 habit of using its lower lip as a foot. The foot is some- 



FIG. 194. MUSSEL. 



A, left shell and mantle flap removed.. 



B, section through body. 

 Question: Guided by other figures, 

 identify the parts to which lines are 

 drawn. 



