VENUS 127 



truding through the lobe of the mantle. 1 They correspond in 

 position to the scars on the shell. 



Do you understand by what means the foot is protruded? 



Gills. — These consist of two pairs of thin, striated, some- 

 what brownish organs, a pair lying on each side of the vis- 

 ceral mass, between it and the lobes of the mantle. 



1. Each gill extends from the wall that separates the two 

 siphons, anteriorly and dorsally to a point nearly opposite the 

 beaks of the shell, and is attached by its dorsal margin only. 



2. Each outer gill is attached along its dorsal border to 

 the corresponding mantle lobe on the outer side. The inner 

 gills, besides being attached to the dorsal margins of the 

 outer gills, are on their inner sides attached to each other and 

 to the visceral mass. (For some distance the inner side of 

 the inner gill lies against the visceral mass, but is not at- 

 tached to it.) 



By this arrangement the space between the lobes of the 

 mantle, which is known as the mantle chamber, is divided 

 into a ventral and a dorsal portion. The ventral portion is 

 much the larger, communicates with the ventral siphon, and 

 because the gills hang into it, it is known as the branchial 

 chamber. The dorsal chamber is known as the cloacal cham- 

 ber. The siphons are frequently referred to by names cor- 

 responding to the chambers with which they communicate. 

 The minute structure of the gills will be studied later. 



3. Place a little powdered carmine on the gill of a speci- 

 men that is submerged in sea water and see what becomes 

 of it. 



Labial Palps. — These consist of a pair of rather small 

 triangular flaps on each side of the visceral mass. 



1. The two outer palps are united above the mouth, which 



is situated just posterior to the dorsal border of the anterior 



1 The anterior foot muscles are sometimes called protractors, and 

 the posterior foot muscles retractors. Both are actually used to retract 

 the foot. The greatly retracted foot may be pulled slightly forward by 

 the anterior muscles, but the mechanism of protruding the foot is very 

 different. 



