25. 



be ruptured, but with these continuous oa-vities the pressure must 

 necessarily remain nearly equal in all the plates. 



Before bringing the discussion of the structure of the 

 gills to a close, reference will be made to Kellogg's opinion 



that the plates of each gill are alternate aiid not opposite. 



His opinion is based on horizontal sections taken 

 below the upper longitudinal muscle, in which position the inner 

 edges of the plates are alternate. This seems to be due to bend- 

 ing caused by the pressure of blood in them. Horizontal sections 

 through the bases of the gills, or Just beneath the upper longitud- 

 inal muscle, as well as whole specimens^ show the plates to be op- 

 posite as a rule. 



It will be remanbered that the gills are suspended by 

 muscular membranes, which probabljr represent folds of the body 

 wall. They are composed of wide plates that are sufficiently 

 V7ide to span the space between the foot and the mantle and, behind 

 the foot, unitedl^r to span the Tnan tie- chamber itself. Anteriorly 

 they gradually diminish in size and finally disappear. Poster- 

 iorly they are attached to the wall that separates the inhalent 

 apid exhalent siphon. The mantle chamber is thus divided by a 

 movable partition, into a ventral chamber, open.i'ng througli tlie 

 inhalent siphon and a dorsal chamber, opening through the exhal- 



