STRUCTURE OF THE LAMELLIBRANCHIATE GILL. 305 



(iii.) Notice that the hooks of adjacent tentacles inter- 

 lock to form the inter-tentacular junctions. 



(iv.) Draw the section. 



4. Examine a section which has cut the inter-lamellar 

 junctions. (Fig. 153.) 



(i.) Notice that the inner ends of the outer and inner 

 halves of each tentacle are united (Fig. 153, A), and the 

 cavities (e, e) of the two sides are continuous across 

 the neck (), thus formed. 



(ii.) The chitinous linings of 

 the two divisions of the tentacle 

 line only the outer ends of this 

 cavity (/), and do not extend 

 into the central portion. 



(iii.) Draw the section. 



FIG. 153. Transverse section of four 

 gill-tentacles of Mytilus, through the inter- 

 tentacular and inter-lamellar junctions. 

 (From Peck.) 



A, B, C, D, E, F, and G. as before. 

 H. Inter-lamellar junction. J. Cavity 

 of the inter-lamellar junction, continuous 



with the tentacular cavity E. 



FIG. 153. 



III. The Gill of Unio. 



Remove the gills from one side of the body by cutting 

 their attachments to the mantle and body ; place them in 

 water for examination. Each of the four gills is now seen 

 to be a flat plate, with a nearly straight dorsal margin by 

 which it is attached to the body, and a slightly curved 

 ventral margin, which is free. 



a. Examine the dorsal margin of one of the gills, and 

 note that it is made up of two parallel plates, the two 

 lamella?, which are united at intervals by cross partitions, 

 the inter-lamellar junctions. 



