Shells of Bivalves. 8 1 



shells (Lingula) of Australia, or by tooth-like hinges, 

 as in the lamp-shells (Terebratu/a), and there are 

 several muscles for opening and others for closing 

 the valves. The mantle in Brachiopoda is full of 

 blood-spaces, which are the only breathing organs in 

 these animals, and there is said to be a heart lying on 

 the stomach for driving on the blood. Some anato- 

 mists dispute the presence of a heart, and claim that 

 the blood is impelled through the body by ciliary ac- 

 tion alone. 



The larvae of Brachiopoda are freely locomotive 

 and possess eyes and ear-sacs, but the eyes disappear 

 in the fixed adult in which the ciliated head lobe of 

 the embryo becomes converted into the basis of the 

 arms. These arms are long and hollow, usually spiral 

 and clothed with tentacles, and their to-and-fro mo- 

 tions cause currents which bring the food within the 

 reach of the mouth of the stationary animals. 



CLASS II. Lamellibranchiata. More familiar to 

 us are the representatives of the second great group 

 of molluscs, oysters, mussels, cockles, &c. These 

 are easily recognised by their bivalve shells, and by 

 the two-lobed mantle under whose folds are the gills 

 or breathing organs arranged in layers or lamellae. 



The freshwater mussel, or the large Mya or clam, 

 easily found along our coasts buried in the sand, out 

 of which the tips of their long siphonal tubes project, 

 are good examples. The shell of one of these ex- 

 hibits to us a beak or point on each valve, and is 

 marked by numerous lines parallel to its margin ; the 

 inner surface also differs in texture from the outer, 

 being whiter and often exhibiting a mother-of-pearl 



