298 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



2. About half way between the foot and the shell the 

 ventral border of the mantle is indicated by a horizontal 

 line or fold (Fig. 149, in) upon the side of the body. 



3. Below the posterior portion of this ridge or fold, 

 notice that the body wall of the embryo is thrown into 

 undulations, so as to form a series of two, three or more 

 rounded prominences (Fig. 149, gi), the rudimentary gill 

 tentacles. 



(i.) The epithelium of these prominences is continuous 

 with that of the general surface of the body, but much 

 thicker, and is made up of a single layer of large cells. 



(ii.) Above the base of each tentacle notice a loose 

 mass of rounded mesoderm cells. 



b. Find an embryo considerably more advanced, in 

 which the two valves have grown downwards so as to 

 cover up the abdomen and gills, and thus form a true 

 mantle cavity. Place it upon a slide in water, and ex- 

 amine the gills as they are seen through the side of the 

 transparent shell. 



1. Each gill is now made up of a series of tentacles, 

 arranged side by side, but not united to each other ; their 

 ventral ends are free, and their dorsal ends are attached 

 to the side of the body. 



2. The thick layer of epithelium which covers them 

 may be traced down one side of each tentacle to the tip, 

 then around and up on the other side to the point of 

 attachment, where it passes to the adjacent tentacle. 



3. The outer surfaces of the tentacles are covered with 

 cilia. 



4. Each tentacle is a hollow tube, closed below ; and 

 blood corpuscles may occasionally be seen in the cavities 

 of the tentacles. 



II. The Gill of Mytilus. 



