94 MOLLUSC A. 



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-w^ater 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 tw^o outer palps are united above the mouth, which 

 is situated just posterior to the ventral border of the anterior 

 adductor muscle, to form a small fold that corresponds in posi- 

 tion 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 wdth a drop of sea-water, and examine with a low 

 power of the microscope. Notice: 



1. 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.^ 



The filaments are joined together laterally by series of bridges 

 (you will see them later), the inter-filamentar junctions, 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 tw^o 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), parallel to the filaments, and at 



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

 where the inter-filamentar junctions are small and well marked, and the 

 inhalant ostia are correspondingly large and distinct. 



