VENUS MERCENARIA. 117 



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 visceral 

 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 vis- 

 ceral mass, (For some distance the inner side of the inner gill 

 lies against the visceral mass, but is not attached 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 chamber. The siphons 

 are frequently referred to by names corresponding to the cham- 

 bers 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 specimen 

 that is submerged in sea-water and see what becomes of it. 



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

 angular 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 

 adductor muscle, and form a small fold that corresponds in 

 position to an upper lip. 



2. The two inner palps likewise unite to form a fold corre- 

 sponding in position to an under lip. 



Make a drawing showing the arrangement of the soft parts. 



Structure of a Gill. — Cut off a piece of the edge of a gill, put 

 it on a slide with a drop of sea-water, and examine with a low 

 power of the microscope. Notice: 



