MOLLUSCA. 193 
Monomyaires. This distinction, however, he has not at- 
tended to with care, as in his family Mytilacées, 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 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 Anomiz ; 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 
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