394 GENERAL STRUCTURE OF LAMELLIBRANCHIATA. 



934. The foregoing description will apply, with slight varia- 

 tions, to the structure of almost all Lamellibranchiata ; but we 

 have now to notice two organs, which are absent in some, and in 

 others more largely developed than in the present instance. At 

 the left side of the figure, projecting beyond the edge of the 

 mantle, is seen the foot ; a fleshy muscular organ, somewhat 

 resembling the tongue of higher animals, and not containing any 

 hard support, or being protected by any envelope. This foot, 

 which is the only special locomotive organ possessed by the 

 Mollusca of this class, serves a great variety of purposes ; some- 

 times enabling the animal to leap with considerable agility along 

 a hard surface, sometimes being used to bore into the sand or 

 mud, and sometimes only serving to affix the animal to some 

 firm support. From the base of this foot there proceeds, in the 

 Mussel and its allies, a band of hair-like filaments, forming 

 what is called the byssus. These sometimes exist in great 

 abundance, and serve, by being fixed by their extremities to the 

 shore or bottom of the sea, to anchor the shell, and yet to allow 

 the animal considerable freedom of motion within certain limits. 

 Frequently the byssus is altogether absent. 



935. The Lamellibranchiatahave usually more power of locomo- 

 tion than the other Acephala. Some of them, however, are at^ 

 tached to one spot during all but the earliest period of their lives. 

 Others adhere by the byssus, or by the foot, by which they obtain 

 a certain range; and others are free during the whole of their lives, 

 swimming and leaping with considerable agility. In these move- 

 ments some of them appear to be directed by powers of sight ; 

 and in these are perceived small red spots at the edges of the 

 mantle, which are believed to be eyes. They do not appear to 

 have much choice of food ; nor are they provided with any other 

 means of obtaining it, than the ciliary action, which introduces 

 constant currents of water into the mouth. In general they do 

 not attain any great size, but they are on the whole larger than 

 any Mollusks except the Cephalopoda ; and a few species attain 

 considerable dimensions, a Pinna having been known four feet 

 long, and a Tridacne (Giant Clamp-shell) having been known 

 to weigh 600 Ibs. They are distributed over the whole globe, 



