XXI. 



OYSTERS. 



309 



mantle-edges close across the lower part, s.s. But the end 

 of the shell-fish which lies to the right in the figure, and 

 which is then uppermost, shows two tubular openings, one 

 above the other. Water is constantly drawn in at the 

 lower tube, i.s., passes over and through the gills, and 

 makes its exit through the upper tube, e.s. 



It is an excellent exercise in observation and interpre- 

 tation for the young naturalist to compare two such forms 



FRESH -WATER MUSSEL. 



o.(j., outer gill ; i.g., inner gill ; mo., mouth ; m., muscles for closing 

 shell ; ma., mantle ; s., shell,/., foot ;h., position of heart ; e.s., exhalent 

 siphon, whence the water passes out from the gill-chamber ; i.s., inhalerit 

 siphon, where the water enters. 



The left valve of the shell has been removed, and the mantle cut away 

 along the dark line. 



of life as the mussel and the oyster and to trace their re- 

 semblances and their differences. 



Now the eggs of the mussel when they are shed become 

 lodged in the chambers of the outer gills. Here they de- 

 velop into embryos so unlike the parent that they used to 

 be regarded as parasites. They are minute bivalve shells, 

 with triangular valves. The hinge runs along the base of 

 the triangle, while the apex is curved round into a strong 

 toothed beak. The small fry remain for a long time in 



