500 PHILOSOPHY OF ZOOLOGY. 



or toothed, and corresponds in position with the back of 

 the animal. The connection of the two valves is secured 

 by the intervention of an elastic horny ligament, the office 

 of which is to keep the valves open. It is either external 

 or internal. The valves are closed by means of adductor 

 muscles, intermixed with tendons, and, passing transversely 

 through the animal, adhere to the corresponding places in 

 the inside of each shell. By the contractions of these 

 muscles the free edges of the valves are brought into con- 

 tact, at the same time that the ligament is compressed or 

 stretched, according as it is internal or external. The num- 

 ber of muscular impressions is employed by Lamarck in 

 the division of the Bivalvia into two orders, Dimyaires and 

 Monomyaires. This distinction, however, he has not at- 

 tended to with care, as in his family Mytilacees, which he 

 includes in his second order, or those having one adductor 

 muscle, there are obviously two adductor muscles, although 

 the one is certainly much larger and more complicated than 

 the other. Besides these impressions of the adductor mus- 

 cles, there are others connected with the foot and byssus. 

 The cloak lines the inside of the shells. In some cases it is 

 entirely open, when the border corresponding with the free 

 margin of the shell is thickened, and more or less fringed 

 with contractile irritable filaments. In other cases, the 

 cloak in front is more or less united, and even forms tubu- 

 lar elongations, which are termed syphons. 



Locomotion is denied to many species of this order. 

 Among these some are immoveably cemented to rocks and 

 stones, as oysters ; a few are attached by a cartilaginous 

 ligament, as the Anomiae ; while others are fixed by means 

 of a byssus. Tliis last organ consists of numerous filaments 

 issuing from a complicated apparatus in the breast, con- 

 nected with a secreting gland, and with the shell, by the in- 

 tervention of tendinous bands. The^o^ is seated a little 



