STRUCTURE OF THE LAMELLIBRANCHIATE GILL. 307 



into view. Notice the dark lines, more numerous than, 

 but parallel to, the partitions. These are the gill tentacles. 



(i.) Crossing these at right angles, and two or three 

 times as far apart, a number of parallel, brownish, gran- 

 ular lines, the inter-tentacular junctions. 



(ii.) In each of the meshes of the rectangular grating 

 which is formed by the intersection of these two sets of 

 lines, notice a rectangular aperture with rounded ends, 

 the external opening of the inhalent ostium. 



(iii.) Note, by focusing up and down, that each of these 

 is continuous with one of the irregular openings already 

 noticed. 



f. Make sketches showing these points. 



g. Turn the specimen over to examine its external sur- 

 face ; wash and stretch it as before, and examine it with a 

 low power. 



1. Notice the fine parallel vertical lines, the edges of 

 the gill tentacles. 



2. Between the tentacles are vertical channels or gut- 

 ters, each of which is covered by two rows of large and 

 very active cilia, which project from the edges of the ten- 

 tacles, and meet over the grooves. 



3. Place a little finely divided carmine upon the speci- 

 men, and notice the ciliary currents along the furrows. 



h. Wash the specimen ; gently cover it with a glass 

 cover, and examine it with a high power.' 



1. Focus so as to bring the outer surface into view, and 

 notice the rows of cilia along the edges of the tentacles. 



2. Focus a little deeper, and notice the double row of 

 chitinous rods inside each tentacle. 



3. Running across the spaces between the tentacles are 

 the fibrous inter-tentacular junctions. 



4. Between the tentacles are the apertures of the inha- 

 lent ostia, situated at the bottoms of the furrows. 



