VENUS MERCENARIA. 119 



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

 fiipoinents. 



(6) The ciha 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 ? 



Draw 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 

 are correspondingly large. By pressing the gill the inter-fila- 

 mentar junctions can be pulled apart. 



Study prepared sections of the gill of Venus and notice; 



1. Lamellae. 



2. Inter-lamellar junctions. 



3. Water tubes. 



4. Filaments. 



5. Inter-filamentar junctions. 



6. Cilia. 



7. Inhalant ostia. 



8. Blood spaces. 



9. Chitinous rods. 

 Draw. 



Understand the direction taken by water in passing from the 

 branchial to the cloacal siphon. AVhat makes the water move? 



Labial Palps. — The positions of these organs have already 

 been noted. 



1. Examine a piece of the palp with a microscope, and notice 

 that the side turned toward the adjacent palp is thrown into 

 ridges and grooves, and is densely ciliated. 



2. The space between each outer and inner palp is con- 

 tinuous with the ''corners" of the mouth. The free margins 



