STRUCTURE OF THE LAMELLIBRANCHIATE GILL. 303 



5. These pads are covered with large cilia which are 

 hooked at their free ends, and the hooks upon the pads 

 of adjacent tentacles interlock, thus forming the inter- 

 tentacular junctions. 



6. Since the ciliated junctions of the opposite sides of 

 the tentacle are opposite each other, a line of junction 

 extends along the surface of the gill, at right angles to 

 the tentacles, and the surface of the gill is thus made up 

 of a rectangular grating, the vertical sides of the openings 

 being formed by the tentacles, and the horizontal ends by 

 the junctions. 



7. The spaces thus bounded (c, c, c) are the incur- 

 rent ostia, through which water passes into the space 

 between the lamellae. 



8. Draw the tentacles, as seen in a surface view. 



d. Embed a portion of a gill which has been hardened 

 in chromic acid, and cut out and mount a number of 

 transverse sections. Examine these with a high power. 



1. Examine a section which has passed through the free 

 portion of the tentacles, that is the portion which is not 

 attached to adjacent tentacles either by inter-tentacular or 

 inter-lamellar junctions. 



(i.) The tentacle, when thus seen in section, is shaped 

 somewhat like the sole of a human foot (Fig. 152, ', a') 

 and consists of a central cavity (e) and a wall of epi- 

 thelium. 



(a.) The layer of epithelium is thin over the sides and 

 inner surface of the tentacle, but the free end, that which 

 forms the outer surface of the lamella, is covered with a 

 thick layer of large cells. 



(6.) These cells carry four bunches of large cilia (d, d) 

 which project over the space (c) between the tentacles, 

 and in the living animal cause the branchial currents in 

 the water which bathes the gills. 



