THE GLAM AND OTEER BIVALVEB 8SELL-FISH. 67 



ocean-world. Nothing which lives has been made in Tain. 

 The world would not be what it is without oysters. 



We can procure oysters at the fish-market which have 

 been opened, and placing one remaining in the lower shell 

 in water, we can, with the aid of Figs. 66-68, examine 

 its structure. Fig. 66 shows the outline of the animal. 

 We can see the edge of the mantle, which sends tentacles 

 out at quite regular intervals. We 

 also sec the large single muscle {M) 

 which keeps the shell closed. As 

 we said, the oyster has no siphon, 

 but the water enters in through the 

 open shell, and passes in towards 

 the mouth (m). The water is set 

 in motion by the moving to and fro 

 of the tentacles. The oyster's food 

 consists of very minute plants and 

 animals, and the minute young of 

 sponges, worms, shell-fish, etc., all 

 too small to be seen without a mi- 

 croscope. The water thus laden 

 with food flows into the mouth, 

 and the food is digested in the 

 stomach (Fig. 67, st), aided by the 

 secretions of the large dark liver {I). 

 The sand and mud in the sea- water 

 and the minute shells and other 

 refuse pass out through the intes- 

 tine (i), a very long tube which 

 ends at v. 



The fresh sea-water also bathes 

 the gills ((/), which are the breath- 

 ing organs. The water passes up 

 between the leaves of the gills, entering the upper gill- 

 cavity through a large number of openings or canals be- 

 tween the gills, and then passes out into an open space 

 above the gills (Fig. 67, si). The object of breathing is to 



Fio, 



Section through the 

 oyster along the line o of Fig. 

 66 (enlarged twice), n'a", dor- 

 sal and ventral branches of the 

 £interior aorta in section; br, 

 artery to gills; c, connective 

 tissue; g, gills in section; g', 

 internal cavities of the gills; 

 ge, egg-gland; ii, cross-sec- 

 tions of the intestinal tube; 

 Z, hver; mf, mantle; sfo, su- 

 prabranchial or water spaces 

 above the gills; st, stomach; 

 ■DC, vena cava.— After Ryder. 



