MOLLUSCA. 193 



J 



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 fring- 

 ed with contractile irritable filaments. In other cases the 

 cloak hr 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. This last organ consists of numerous filaments 

 issuing from a complicated apparatus in the breast, connect- 

 ed with a secreting gland and with the shell by the inter- 

 vention of tendinous bands. The foot is seated a little to- 

 wards the mouth, is usually tongue-shaped, capable of con- 

 siderable elongation, with a furrow on its posterior surface. 

 This organ, where a byssus is present, is considered as 

 employed in spinning and fixing the threads. When there 

 is no byssus, it either acts as a sucker, enabling the animal 

 to crawl among the surface of bodies, or as a paw, to dig 

 holes in the sand or mud. None of the species can float in 

 the water. They either crawl or leap, the last kind of 

 motion being effected by suddenly opening and shutting the 

 valves. In securing a residence, some of the species bore 

 into different substances by means of a rotatory motion of the 



