1 8 " MOLLUSC A. 



cut across that portion of the mantle, on the upper side, 

 which Hes loosely between the large muscle and the 

 body, as in Fig. 5. 



This operation exposes the thin membrane, which 

 covers the cavity occupied by the heart, and calls at- 

 tention to the close attachment of the outer lamina of 

 the outer gill and the inner side of the mantle. 



It also gives an inside view of the gills, showing that 

 they form a ridged and grated bottom to a cavity run- 

 ning along their entire length. 



A little blueing, again placed on the outside of the 

 gills, will now be seen to pass into them, and come out 

 into this cavity with considerable force from a series 

 of tubes in the interiors of the gills. (Figs. 2, og, 5, 6.) 



In order to expose the gill cavity fully, sever the 

 connection of the outer side or lamina of the outer gill, 

 with the mantle (so as to leave a portion of the base 

 of the gill still attached to the mantle) all around the 

 large muscle, and also anteriorly, until the mantle can 

 be turned back and disposed of on either side, as in 

 Fig. 5, m\ 



Then, by tilting the gills over towards the convex 

 side, it will be seen that each one is made up of two 

 sides or laminae, united to each other on their outer 

 free edges, but not on their inner edges ; that here 

 the laminae of each gill are spread apart, and connected 

 only by fleshy partitions, while the adjacent laminae of 

 neighboring gills approach each other, and join, so as 

 to form three fleshy ridges running the whole length 

 of the gill cavity. (Figs. 6, 7.) 



The interior of each gill is divided by numerous 

 fleshy partitions into a series of tubes opening into the 



