394* GENERAL STRUCTURE OF LAMELLIBRANCHIATA. 



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, when 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. 



1019. The Lamellibranchiata have usually more power of lo- 

 comotion than the other Acephala. Some of them, however, are 

 attached 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 movements some of them appeaT to be directed 

 by powers of sight ; and in these small red spots, which are be- 

 lieved to be eyes, are perceived at the edges of the mantle. 

 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 Cephalo- 

 poda ; and a few species attain considerable dimensions, a Pinna 

 having been met with four feet long, and a Tridacna (Giant 

 Clamp-shell) having been known to weigh 600 Ibs. They are dis- 

 tributed over the whole globe, principally frequenting the shores 

 or shallows. Each region has certain species peculiar to it, or 

 most abundant in it ; and there are few which are not limited to 

 one hemisphere. The temperate zone appears as favourable to 

 the development and multiplication of some species as the torrid 

 zone to others ; but the largest kinds are only found in warm 

 latitudes. 



1020. In regard to the subdivision of the Lamellibranchiata 

 into Orders and Families, great difficulty is felt by those by whom 

 this group has been most studied. By some, the presence of a 

 single or double adductor muscle has been taken as the ground 

 of the primary division ; but for the reason already stated, this 



