VENUS MERCENARIA. 95 



right angles to the inter-filament ar 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, unthont pressing it, under a cover- 

 glass in a drop of sea-w^ater and observe with a low power: 



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



(b) 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. 



(b) The cilia on the sides of the filaments. These are of 

 two kinds: (1) Surface cilia that form currents of water along 

 the filaments. These will be seen waving back and forth, or 

 if still moving rapidly, apparently moving along the sides of 

 the filaments. (2) Deeper cilia that are down between the 

 filaments and can be seen by changing the focus. These move 

 at right angles to the others, and apparently become longer and 

 shorter. Why ? 



Draiv a surface view of a piece of a lamella. 



Examine a piece of the gill of Mytilus for the above struc- 

 tures. In this form the inter-filamentar junctions are small 

 and composed of modified cilia only, and the inhalant ostia 



