MOLLUSC A. 19 



gill cavity above. The bases of these partitions cross 

 the openings of the gill at right angles, from one fleshy- 

 ridge to the next, so that the bottom of the gill cavity 

 thus formed has the aspect of a grating. (Figs. 5,6.) 



Although most observers will be content to stop at this 

 point, it is to be hoped that those having magnifiers and 

 microscopes will try to go farther into the structure of 

 these curious and complicated organs. 



In order to do this, divide a portion of a gill, by cutting 

 the partitions and taking out a piece of one lamina, and lay 

 this under the magnifier in a watch glass, and covered by 

 water. 



The inner side (Fig. 8, r" , /") will be found to be com- 

 posed of a series of vertical tubes with thick walls. 



The vertical tubes are united by smaller horizontal 

 tubes (Figs. 8, 9, ^"+) at regular intervals, so that here 

 again we see a grating presented in the structure, but this 

 time of small meshes. 



If now the focus of the microscope is changed, and the 

 gill is in good condition, and can be stretched open a little, 

 these meshes will be seen to open into semicircular cavities 

 (Figs. 9 and 6'), which show that the vertical tubes lie at 

 the internal junctions of the vertical folds of the outer side 

 of the gill. The infoldings of the outer surface are perma- 

 nent, and each one corresponds exactly in breadth to the 

 space between two internal vertical tubes (Fig. 1 1, r", /"). 

 On the surfaces of these flu tings are numerous smaller 

 vertical folds (Fig. 6') ; and these, the most minute folds, 

 are united by cross-bars, with apertures between, so that 

 here we have still another and a very fine grating, and 

 one admirably suited to strain water of food and all large 

 particles. These are shown from the interior also in Fig. 

 9, eo. The course of the water, then, is evidently through 

 these minute, external gratings, first into the semicircular 

 chamber^ of the interior of the gill laminae, then through 



