VENUS MERCENABIA. 103 



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-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 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), parallel to the filaments, and at 

 right angles to the inter-filamentar junctions. These are called 

 the inter-lamellar junctions. By means of the inter-lamellar 

 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 most readily be done by catching the free dorsal bor- 

 der 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 removed with the forceps. Mount this piece, 

 with the outer surface up, without pressing it, under a cover- 

 glass in a drop of sea-water and observe with a low power: 



(a) Filaments, that run the width of the gill. 

 (6) Inter-filamentar junctions, which form bridges connect- 

 ing the filaments. 



(c) Inhalant ostia. The opening bounded by filaments and 

 inter-filamentar junctions. 



(d) The position of the torn inter-lamellar junctions, appear- 

 ing as indefinite dark stripes running in the same direction as 

 the filaments. 



With a high power observe : 



(a) The chitinous rods that lie inside of, and stiffen the 

 filaments. 



1 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. 



