128 MOLLUSC A 



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 cor- 

 responding 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. 



1. Notice the cilia on the edge and surface of the gill. 



2. The surface is marked by a series of parallel ridges, 

 the filaments, with grooves between them. 1 



The filaments are joined together laterally by series of 

 bridges (you will see them later), the inter filament ar junc- 

 tions, with the pores, inhalant ostia, between them. Each 

 side of the gill is thus composed of a single layer of united 

 parallel filaments, which together form what is known as a 

 lamella. Each gill is composed of two such lamellae, one on 

 each side. These lamellae are united at intervals by bridges 

 that run the whole width of the gill (dorsal to ventral) , par- 

 allel to the filaments, and at right angles to the interfilamen- 

 tar junctions. These are called the interlamellar junctions. 

 By means of the interlamellar junctions, the space between 

 the two lamellae is divided into a series of water tubes. The 

 openings of these tubes into the cloacal chamber may easily 

 be seen after the cloacal chamber has been cut open. 



3. Separate a small piece of one lamella from the other. 

 This can be done most readily by catching the free dorsal 

 border of the inner lamella of an inner gill with the forceps, 

 and either tearing off a piece or freeing it by cutting with 

 scissors while it is being pulled with the forceps. Mount this 

 piece, with the outer surface up, under a cover glass in a drop 

 of sea water and observe, with a lower power, the following: 



1 The general surface features are especially easily seen in Pecten, 

 where the interfilamentar junctions are small and well marked, and the 

 inhalant ostia are correspondingly large and distinct. 



